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Fueling the Carb


Before you consider digging into the carburetor, you must ascertain the “health” of the fuel delivery system.  A fuel pressure gauge (either hand held or plumbed inline) is required for this exercise. Always get the powerplant warmed up to operating temperature prior to performing this test. (In fact, in most tuning operations this is a mandatory exercise).
Holley carbs can normally withstand a max. fuel pressure of 7-1/2 pounds per square inch (psi) before the needle and seat assemblies are overcome.  Any pump reading 9 hot under 4 psi indicates a faulty fuel pump, although some vehicles will function normally with as little as 3 psi hot.  In the event that your fuel pump is suspect, consider adding a heavy-duty, high capacity fuel pump.  In addition, when engine speed approaches and exceeds 6000 rpm, the fuel pump pushrod is prone to floating (just the same as valve float).

B & B Performance offers a lightweight fuel pump pushrod with hardened ends to solve this problem. Original equipment Holley carburetors are sometimes fitted with bronze-type filters inside both fuel inlet fittings.  Although these filters perform an admirable job of filtering the fuel before it actually enters the carburetor, they are a source of massive fuel restriction and when filtering, must not be neglected, as the newly filtered dirt will slow the fuel delivery down even more. Since they are in a location that is past the normal fuel pressure gauge pickup point, they are very seldom blamed for a loss of fuel pressure. In order to combat the problem, simply remove and discard the filters as well as the accompanying check springs that hold the filters in place.  Install a large inline filter before the fuel pump and be sure to inspect the assembly on a regular basis. Fuel filters can cause a nightmare if they decide to fowl up at the wrong time.  For a few bucks in the grand scheme of things don’t skimp on fuel filters, you will kick yourself.


The most common Holley performance carburetors use a fully adjustable needle and seat setup.  This is used to raise or lower the fuel level in the bowls and is actually accomplished by setting the float level. Carbs with this feature are equipped with a removable sight plug on the side of the bowl. Simply remove the sight plug and check the level. Fuel should gently seep out of the sight plughole.  If fuel pours out, the float level is too high.  If the fuel cannot be seen and does not seep out, the float level is too low.  The vehicle should be parked on a level driveway for this part of the engine power tune-up. In order to adjust the float level, simply back off with the tip of the screwdriver slot-equipped lock screw and then set the level with the adjustment nut.  Re-tighten the lock screw once the adjustment has been finalized.  If the float mechanisms are not externally adjustable, the float level must be set with the fuel bowl removed from the carburetor. This will take a steady hand and some careful adjustment.

Once the bowl is removed, invert the assembly and align the float so that the top (now the bottom since the bowl is upside down) of the float is parallel to the top of the bowl (not at an angle). The tabs may have to be bent, as this is the only method of adjustment available. Two common problems with carbs are internal fuel leakage and constantly “sinking” floats.  To cure an internal fuel leak, look no further than the needle and seat assembly. Occasionally, a minute piece of debris can jam the assembly causing it to hang open.  A float assembly that leaks fuel into it, almost always causes a ‘sinking float’. The ruptured float fills itself with fuel and all the qualities of the floats one purpose are lost.
While repairs are possible, the “fix” is hardly worth the effort unless you have no other option. Simply pick up a new float from your nearest speed shop around the corner.

Holley needle and seat assemblies are available in a variety of sizes and configurations; however, the vast majority of high-performance applications (carburetors featuring external float adjustment hardware) can make use of the 0.110-inch Viton-tipped setup (P/N6-504).  If your application uses fuel additives that contain alcohol, benzene or acetone, you should be using a steel needle and seat, while larger carbs may use the 0.120-inch or 0.130-inch needle and seat units.  Holley part numbers are as follows: 0.110-inch = P/N 6-500, 0.120-inch = P/N 6-502 and 0.130-inch = P/N 6-515.  If replacing needle and seat adjustment hardware (the top lock screw and setting nut utilized for float adjustment), always be sure to use genuine Holley brand components.

Quality is the main reason for using the Holley hardware.  Many of the generic parts are so poor in quality that they will destroy the fuel bowl threads only after a few adjustments. Stick to OEM parts to avoid disappointment. Looking4spares is your Part Find, parts locator service that will find any new parts or used parts that you are looking for. “Looking4spares” has never been easier.

Visit our website at www.looking4spares.co.za or contact the “PART FIND” Call Centre on  0861 77 77 22  or  e-mail: This email address is being protected from spambots. You need JavaScript enabled to view it.


Main jetting is often the cause of confusion in carburetors. The ‘as delivered’ jetting from Holley is very close to the optimum for the majority of hi-performance applications. Certain carbs require stagger jetting, but for the most part, the jets delivered with your carburetor should suffice your tune-up ‘baseline’. Holley carbs are calibrated at sea level at 70 degrees F.  For every 35 degrees F increase the air-inlet temperature or 2000-foot increase in altitude, the jetting should be decreased one jet number. This amount in jet size amounts to approximately a 0.002-inch decrease in the size of the orifice.

Carburetors that are stagger jetted or jetted proportionately. As an example, if the primary jets feature No. 72 jets and the secondary jets feature in No. 74 jets and your wish to increase jetting by one number, then you should increase the primary jets to No. 73 and the secondary jets to No. 75.

If you are contemplating re-drilling the jets, don’t even consider it!  The jets are precise pieces of equipment that are broached to the correct size by Holley.  Drilling the jet is simply not accurate and in fact, ruins the jet. Why would anyone even think of doing this, you will never attain the perfect polished size, for a few bucks hit the speed shop.

Certain Holley carburetors, such as 4160 versions, do not have replaceable jets on the secondary side.  In place of the jet block is a special metering plate.  The only way to change the orifice size of these plates (when re-jetting) is to replace the entire plate.  Although special plates are generally available from Holley for this purpose, finding them can sometimes prove troublesome.  A much easier method is to convert the carburetor to a 4150-type assembly, which uses replaceable jets.  The proper kit for making this changeover is available from Holley part number 34-6. Again use of a dyno will make the tune-up in this department a breeze.


Occasionally, a tuning problem that appears to be fuel starvation is diagnosed during vehicle testing.  Before placing the blame on the fuel delivery system or the carburetor, give some consideration to the fuel inside the fuel bowl.  A hard-leaving drag race combination may actually be uncovering the jets in the fuel bowl, thus giving the illusion of a loss of fuel or a loss of fuel pressure.  Because the front of primary fuel bowl features jets that are at the rear of the bowl, the fuel does not “run away” from the jet.

On the other hand, secondary or rear fuel bowls have jets situated ahead of the bowl and fuel, which creates the momentary fuel loss problem.  This can be cured easily with the addition of a set of Holley jet extensions (P/N 26-21).  These simply fit over the jet body in the fuel bowl, extending to the rear of the bowl where gasoline is always present.  In certain applications, the jet extension will interfere with the float.  In order to solve this new problem, use the nitrophyl-type float assembly.  Notch the float for clearance and then fill the notch with a gasoline-resistant epoxy.


The fuel “enrichment” circuit of a Holley carburetor is based on a power valve system.  In essence, this system will add approximately 10 jet numbers of fuel to the engine under demand.  These particular power valves are located in the metering block of the carburetor, so changes are relatively simple and straightforward.

In order to determine the proper power valve for the application, start the engine with a vacuum gauge attached to the intake manifold.  The correct power valve must correspond to the engine idle vacuum registered on the vacuum gauge.  Each and every Holley power valve features a stamped-in number that corresponds to the valve opening point.  As an example, a #65 power valve will open 6.5 inches of manifold vacuum, while a #35 power valve will open at 3.5 inches of manifold vacuum.

The correct valve should open one to 1.5 inches HG below the manifold reading at idle.  In other words, if you car has a vacuum reading of eight inches at idle, the power valve should open at 6.5 inches HG (with a #65 power valve).

When the throttle is hit wide open on the engine, the vacuum level drops, which allows the power valve to open.  This is a very critical step in tuning the carburetor.  If the engine utilizes a long duration cam (and consequently, has very low manifold vacuum readings), the power valve could easily be opening during idle – dumping raw fuel into the engine (in massive amounts no less!).  This leads to a very erratic idle and absolutely no off-idle throttle response.

Altitude plays a very important role in the selection of power valves.  The circuit must be “readjusted’ to compensate for the decreased available intake vacuum.  It is recommended that the power valve timing or size be reduced 1.5 inches for every 3000-foot increase in altitude above sea level.  This adjustment is done simply to compensate for the lack of available “air”.  Keep this tip in mind whenever the altimeter soars – even if you live in an area near the ocean.

Holley carburetor power valves are available in wide variety of combinations. Make sure you get the right application.
The high-performance-type units are called “single-stage” assemblies and are listed under part number 125-25 through 125-105 (2.5 HG through 10.5 HG opening points with 1.0-inch HG steps between the respective part numbers).  In addition, a set of high-flow power valves are available under part number 125-165 and 125-1005.  These components are rated at 6.5 inches and 10.5 inches Hg respectively, and would make a great choice for mild cammed, high manifold vacuum Chevy’s.


The accelerator pump circuit of a Holley carb is one of the most crucial in a hi-performance engine. Tune-ups in this area are very important and much like ignition curves, the accelerator pump circuit should be tuned to the specific engine.  All Holey performance series carbs (with the exception of drone assemblies found on some tri-power applications) feature at least one accelerator pump. So don’t think that you can just use a similar Holley carb that you have robbed off another engine that worked out for you at your last meet and make a few quick modifications. That would be considered pure desperation and leave you back at square one.

Almost all Holley carburetors are fitted from the factory with a standard volume accelerator pump.  These pumps are adjusted by opening the carburetor to wide open throttle or WOT, with the engine turned off.  The pump lever should not bottom out in the accelerator pump housing, but should feature at least 0.015 inches of extra travel.  Don’t be tempted to tighten the accelerator pump screw at the pump lever spring in anticipation of increasing pump travel and volume!
This spring is pre-set and, in almost all applications, it should not be touched!

Highly modified vehicles (particularly those equipped with an automatic transmission) can sometimes develop a severe stumble off idle.  The correct ‘method of cure’ for this problem involves increasing the accelerator pump shot.

If tuning (such as revising the pump cam timing or increasing the shooter size) doesn’t work, you may have to increase the size of the rear accelerator pump.  Holley sells a 50cc accelerator pump kit (often referred to as the “REO kit”) for this application.  This part (Holley part number, P/N 20-11) includes the body, special diaphragm, arms, cams and all mounting hardware.  When installed, it should easily sole the off-idle stumble, but you may only need to “play” with original pump cam timing, as well as accelerator pump shooter size. If you stick to these steps and apply them as closely as possible and have drive in you, perseverance will win the day. Perfection is what you are striving for when getting stuck in to a Holley tune-up.


Accelerator pump shooters are attached to the main body of the carburetor in the venture area and are held in place by a Phillips-head screw.  These components are used to tune off-the-line acceleration.  If the initial acceleration produces a hesitation and then picks up, the shooter size must be increased.  In certain cases, the accelerator pump shooter may be so small (lean) that the engine will backfire during acceleration.  If the shooter is too large, the off-idle acceleration will not be crisp or clean.  Additionally, a shooter that is too large will often create a puff of black smoke during acceleration.  In this instance, the shooter must be replaced with a smaller 9leaner) example.

A very common misconception in regard to shooter sizing is an engine bog or hesitation-just off idle.  Many novice tuners feel that the “bog” was created by an excess of fuel, so they lean out the jetting of the carburetor.  This is completely incorrect !  Although it may initially appear like far too much carburetion, the bog is created by too little fuel. An air/fuel ratio hole has developed in the fuel curve.  Too much air is allowed in the engine as the throttle is cracked open and there is insufficient fuel to cover up this “air hole.”  The solution is rather simple: Keep increasing the accelerator pump shooter size until the bog is cured.  As you can easily determine, no steadfast rules apply to shooter selection.  It is merely a trial-and-error task that can be tuned only via experimentation.

Holley offers many sizes, shapes and configurations.  The two high-performance types include the tube discharge examples and the straight-type end discharge assemblies.  Although quite different in overall appearance, there seems to be little difference in the performance of either hi-performance shooter unit.  All shooters are numbered from 25 through 52 (the stamped-in numbers indicate the drill size of the shooter orifice). While jets cannot (and should never), be re-drilled, shooters are another matter.  Re-drilling the orifice size with a pin-vise drill is common practice, however, the numbering “system” is then thrown out of kilter.  Drill them if you prefer to, but always remember to physically check the orifice size prior to installation.

It should be pointed out that when shooter size is increased beyond 0.40 inch, it is wise to make use of Holley’s hollow shooter screw kit (part number 26-12).  This setup allows increased fuel flow to the pump shooter, ensuring that the limiting restriction in the accelerator pump system is in fact the shooter, not the screw.


A companion tuning aid to the shooters are the replaceable accelerator pump bump cams.  Much like specifically profiled engine camshaft, the carb pump cams have varied lift and duration profiles.  The specific cams are color- coded and each cam has a pair of available mounting positions. Like the shooters, the pump cams require trial and error; the experimentation process of elimination to determine exactly which cam is going to be best suited for you particular application.  Most experienced engine tuners will first tailor the pump discharge nozzle (shooter and then utilize the pump cam assembly to further fine tune the system). The process also involves a lot of patience and time and a dynamometer is probably the answer here again.


To set the idle mixture properly. Fit a vacuum gauge, which can be installed on the engine at a neat position for referencing easily.  Pick one idle mixture screw and call it the “#1” mixture screw.  Set this screw until the highest manifold reading is obtained on the vacuum gauge.  Then proceed to the second mixture screw and set it to the highest reading.  Go back and repeat the process, fine tuning the idle mixture as you go, and repeat the process for a third and final time.
Once the correct idle mixture has been established, turn your attention to the idle speed screw found on the driver’s side of the carb.  In most cases you will find that the vacuum gauge method of setting the idle mixture will have increased the idle speed by quite a substantial margin.

Reduce the speed to a level that allows the engine to idle properly.  Automatic transmission examples should be capable of idling slowly in gear.

In the event that the carburetor has been apart and the engine wont start following a carb rebuild, you can approximate the idle mixture setting by turning both screws all the way in (being careful not to over tighten the screws against the internal seats).  Back the screws out approximately 1-1/2 turns, but always reset the mixture with the vacuum gauge in place.



Almost all hi-performance distributors have mechanical advance system.  The centrifugal advance system is designed to advance the firing of the spark, typically using springs and weights inside the distributor body.  As engine speed increases, the spark for each cylinder must be triggered slightly sooner to allow some time for the full ignition of the air and fuel mixture prior to the piston reaching top dead centre.  In operation, the distributor shaft speed of rotation builds up and the shaft speed increases. The mechanical weights tend to fly outward, stretching the springs more as the engines rev’s increase.  Pins on the weights act against a plate fitted to the base of the dizzy cam.
The further the weights fly out, (hence the name “centrifugal” which is the description of the action), the further the cam position moves which results in advanced timing the firing of any engine fitted with one of these distributors.

In order to check out the mechanical advance system, remove the distributor cap. Hold the rotor by hand and see if you can turn it in the same direction that the distributor shaft normally rotates.  There should be some pressure of the advance springs holding back the movement.  Release the rotor and it should snap back to its original position. “Total timing” is the amount of ignition timing, (in crankshaft degrees), that the engine “sees”.

Now, if you think about a typical vacuum advance-equipped street distributor, you can easily see that the total amount of timing can approach huge levels if not held in check and adjusted accordingly. Look at the variables: Initial advance, centrifugal advance and vacuum advance.  Too much of each, (and it isn’t difficult), can result in 50 or more degrees of total timing, (this would be considered way too much). Initial advance and total mechanical timing can vary from engine to engine, (even those that are seemingly identical), and only your own engine can “tell” you the exact numbers required by trial and error.

Generally spreading, a hi-performance combination can utilize total timing figures ranging from 36-46 degrees. This includes initial and mechanical advance.  Auto transmission equipped vehicles can use more initial advance than their manual stick shift counterparts.  However, the total timing should still be the same.  What is meant by all of this is the mechanical advance will have to be shortened with an automatic transmission in many cases.

While this may sound frightening, in reality, it is quite simple, (depending upon the distributor type that you have selected at the time of the engine modification).  You might be able to shorten the advance curve by increasing the size of the weight stop thus slowing the action down slightly. Other examples may require minor welding of an advance slot, while some aftermarket distributors have provisions to change the duration of the overall mechanical advance curve. It’s just a matter of examining your specific distributor and determining how the mechanical advance weights are limited.  Generally speaking, the mechanical advance should be between 2000-2400rpm for automatic-equipped cars and in the range of 2400-2800 revs per minute for manual gearbox examples. You must go through the motions and identify the best possible variant.


There’s no question about it that the distributor cap and the rotor must be positioned precisely in order to properly and efficiently distribute the spark at the correct time.  There are two areas where cap to rotor fit are critical.  The first is the rotor to distributor cap clearance.  It must be close enough to allow the ignition spark to cross the gap easily, (not too short and not too long). In the second case, the rotor end or point of discharge must line up exactly with the distributor cap terminal precisely when ignition spark fires.  If the distributor cap to rotor blade clearance is too great, the given spark can easily ‘jump’ to the next cap post terminal in the firming order. The same thing can occur if the cap to rotor alignment is off, even if it is only off by only by a small amount.  Not only will this “jumping spark” result in a poor running combo, it can also destroy the engine if operated for any duration under these conditions.  To make it simple, the effect can be the same as far too much initial spark.

Its no secret that it is physically impossible to “move or adjust” a conventional rotor.  But that isn’t the case with the distributor cap.  If you look closely at the cap, you’ll notice that there is a margin of “free play” on the cap mounts.  Most rotor caps can be twisted slightly clockwise or anti-clockwise, even when the mount clips are lined up (but not completely fastened).  In order to check and properly phase the system, use the following process to find the exact rotor to dizzy cap post:

Prepare a “test cap” by cutting a large “viewing window” in the body.  The window should be positioned under the cap posts, but high enough so that you can see the body of the rotor.  It doesn’t matter where (in relation to the firing order) this window is cut.  Simply position the window in a spot that is easy to view on your engine while it is running.

Distributor Cap

  • Mark the distributor cap with a series of index marks that correspond with the exact center of each post in the “window”.  Position the marks at the bottom and top of the window and be sure that the marks are lined up straight.  To make the marks visible on a brown colored distributor cap, use a fine black marker.  If your dizzy cap is black, lightly cut the marks with a fine file and fill the marks up with white Tippex correction fluid or find some white paint for the task.

o Mark the rotor with an index mark (a light cut with a small file works well).  This index mark must coincide with the center of the rotor blade tip.  In order to make the tip easy to view, fill the index mark with Tippex.
o Install a timing light to one of the distributor cap posts in your window.  Set the engine speed so that the rotor appears to be steady. Use the timing light to view the position of the rotor versus the distributor cap post.  As expected the timing light will “slow” the rotor speed so that you can spot the location of the rotor tip in relation to the distributor cap terminal.

Indexed Rotor  

  • If you engine does not have vacuum advance, phase the rotor so that the tip lines up precisely with the distributor cap terminal.  To accomplish this with a conventional dizzy cap, you will have to note the test cap (mark it with Tippex or a felt-tip pen). Replace the test cap with a conventional cap, but make certain that you have placed it in the same position as the test cap.  (Be sure to use the same brand of cap as the “test cap.” (This will ensure that you get exactly the same timing marks). Once you have determined the location and marked the distributor body, you wont have to check the rotor phase again.)  As indicated earlier, there’s a slight amount of play available when installing a distributor cap. In most cases, this play should prove sufficient to phase or index the rotor or distributor cap position.

If the car has vacuum advance, you must take into consideration the advance will change the phasing of the rotor according to manifold vacuum.  A distributor with clockwise rotation (vacuum advance disconnected and plugged) should have the rotor just to the left of the  “target” distributor cap post.  When the vacuum advance is working, the rotor will appear just to the right of the target post. Similarly, a dizzy with an anti- clockwise rotation will be exactly opposite to the above.If there isn’t enough play in the cap to compensate for any misalignment,
there are ways to increase the amount of adjustment.  Many point breaker plates or magnetic pickup plates found inside the distributor can be repositioned slightly. Coupled with the available movement of the cap, you
should be able to properly index the cap and rotor combination.


It’s always a good idea to index the spark plugs, but what is it and what good does it do? Every engine responds differently in many ways to indexed spark plugs. Combustion chamber shapes, piston dome configurations, ignition systems and a host of other combinations influence the spark plug “location” in the cylinder head combustion chambers. In some examples with a large piston dome coupled with a tight fitting combustion chamber, indexed spark plugs are mandatory, only because the close internal clearances “tighten” the spark plug gap every time the piston comes to top dead centre.

In terms of performance, some engine combinations simply respond better to indexed spark plugs than others. The idea of the exercise is to position the spark plugs so that the gap is facing the center of the cylinder or central point of combustion chamber firing area, angled slightly toward the position of exhaust valve.  (That’s the most common arrangement. Some combustion chambers tend to “like” other spark plug gap locations).  Why is this so important?  As the piston approaches top dead centre the ‘charge’ or air/fuel mixture is being compressed. The “mixture” is forced toward the area of the spark plug (and usually this would be towards the exhaust valve area). The true speed of this force inside the combustion chamber is extremely fast. Some experts feel that it surpasses supersonic speeds.  Because of this the spark from the spark plug, the plug should be in a ‘position” to create the best possible flame front. Bear in mind that the spark plug will always burn outwards and away from the electrode stem. Twin spark electrodes plugs are available for certain configurations although if indexed correctly conventional plugs may be the better option.

If you look at a typical side gap spark plug, you’ll note that the electrode can actually block the flame process. On the other hand, if the electrode gap faces the on-rushing air/fuel charge, it stands a much better chance of igniting a flame front thus creating or igniting an explosion from a more central point. In order to index the spark plug correctly, mark the plug insulator body with a black felt marker pen on the side where the ground electrode attaches to the spark plug body.
Instead of rummaging through boxes and boxes of spark plugs in an effort to locate the elusive combination of perfect spark plug threads that match the respective cylinder head threads, use aftermarket indexing copper or ally washers.  These soft malleable washers are available from Moroso and B & B Performance. Moroso kits are supplied with .060-inch, 080-inch and .100-inch thickness washers, while B & B kits feature .010-inch, .021-inch and .031-inch gaskets for tapered seat spark plugs and .064-inch gaskets for flat seat spark plugs.

Due to the soft nature of the copper or ally, along with varied washer thicknesses, it now becomes a simple matter to thread the spark plug into the respective cylinder and tighten it to the point that the index mark is situated in the correct position relative to the combustion chamber as described above.  Just be certain that you do not double up on the washers. They aren’t meant to be used in tandem and using more than one recesses the spark plug lessening it’s reach to the central point where it should be in order to give the best chance for ignition and burn an optimum flame.


Spark plug gaps should always be set with either a Champion plug gapping tool or a high quality feeler guage set.  Although it has been discontinued, one of the best plug gapping tools ever released to the public was the Accel plug gapping tool. If you ever have the opportunity to purchase one of these tools, do so without giving it a second thought; you may not get the chance again!  All spark plug gaps should be set according to the ignition manufacturer’s recommendations and specifications.  (For example, a MSD multiple spark ignition will easily fire a spark plug with a gap of .045 inches. With good quality wires and added spark plug wire insulation, gaps of .065 inch and greater can be fired.)  Naturally, there are some things you can do to your combination that will ‘pick it up’ or intensify it, (from a spark plug perspective). One such trick is a “clipped gap.”

If you are struggling with your “combination” and can’t quite turn the elimination times you thought were possible, consider cutting back the ground electrode on your spark plugs. But in this case, it doesn’t mean a mere file job, it means clipping the spark plug with a pair of wire cutters so that the ground electrode is flush with the side of the center electrode or shorter than the center electrode.  What this does is to increase and create a huge spark plug gap (something in the order of a couple hundred thousandths of an inch).

This technical tip only applies to engines fitted with high powered ignition control systems.  A normal point triggered affair would have trouble starting, let alone running with severely clipped spark plugs. Be “gentle” on the vehicle combination once you’ve clipped the spark plug. By this we mean that you should not drive the car to and through the drag strip staging lanes if possible. Once you fire it up, do your normal burnout stage and make your pass without ‘wasting’ the new set of clipped spark plugs. (They don’t last very long, so depending on your engines degree of performance, add on’s and the fuels you are using all come into account, etc, etc).

In some cases, the “clipped” electrode will pick up your combination as much as a tenth (but like indexed spark plugs, other examples show little improvement). Just be sure that the spark plug wires and the balance of the ignition system are in perfect operating condition. Why? The answer is very simple. The huge spark plug gap creates a mini-flame-thrower inside the combustion chamber. The spark plug wires will eventually self-destruct when this much gap is used, but remember, this is just a one-shot deal to make your elimination time, work out for you. If it doesn’t quite work out on your run, return to your pit lane and replace the plugs with a more conventional spark plug gap arrangement.  But if a one-shot performance is what you are looking for, this trick is one that just might help to turn up the magical elimination times to the number always hoped for. This can only be determined through trial and error, so don’t throw in the towel after a one hit run, it will need all the right combinations to suffice.


Spark plug heat ranges and the terms ‘hotter’ and ‘colder’ are often confused on the part of an engine tuner or a general mechanic that isn’t really affay with building horsepower in the big numbers. The terms are actually used to determine the thermal characteristics (heat rating) of a given spark plug. This is the ability of a spark plug to transfer heat from the firing end to the cylinder head, which in turn transfers the heat into the engines water jacket and then into the radiator / cooling system.

Cold spark plugs transfer the generated heat very rapidly. This type of spark plug normally used in engines with a relatively high combustion chamber and cylinder head temperature (such as racing cars or highly modified street engines). Hot spark plugs transfer heat to the water jackets at a much slower rate keeping the heat in the combustion chamber so that deposits are properly burned away, preventing fouling. In a nutshell, hot plugs are used where combustion chamber temperatures are cool and visa versa.


It’s no secret that ignition wires which feature ‘spiral’ inner circles are the hot ticket to success. These ‘street wires’ not only do a brilliant job of suppressing radio noise interference, but they also have found a stable home in pro racing circles. There’s no question that boom-boom-box sound systems are low down on the priority list when it comes to drag racing and street rod applications, but one thing these all out street rods and race cars do have nowadays is sophisticated on-board electronic computerized systems. Systems like high ticket data gathering equipment (read: on board computer systems), intricate high performance ignition systems, ‘two-step’ rev limit modules and other up-market electronic hardware and software has now become mandatory in modified street cars and all out racing applications in today’s muscle cars.

Almost all of this electronic wizardry can be affected by the same radio frequency (RF) noise that drives the stereo system crazy in a streetcar. When one spark plug wire fires, it induces a voltage into the parallel wire. Using a typical Chevy V8 as an example, the firing order is 1-8-4-3-6-5-7-2. Cylinder 7 fires directly after cylinder 5 and both are situated side by side in the bank numbering system. This is very significant because as number 5 fires, number 7 is just starting the compression stroke. If number 5 and 7 are running parallel, or if they are routed together when number 5 fires, it will induce a certain amount of voltage into the number 7 wire. With the fuel charge already in number 7 (along with a sufficient amount of compression), it fires easily. Chevrolet’s aren’t the only engines plagued with this side-by-side arrangement. As you can easily see, this very advanced and equally unplanned timing can quickly result in misfiring causing serious problems-not the least of which is outright cylinder wall failure. There are solutions to the problems faced and always ways to get around this.


You’ve probably heard about ignition wire sleeves, but how do they work? As spark plug gaps increase and ignition power becomes stronger, the chance of spark leakage through the wire increases. High powered ignition control boxes have become the norm rather than the exception and because of this, almost all wires are susceptible and exposed to high voltage leakage and of course the problem of cross fire. Moroso, MSD and other companies offer ‘spark plug wire sleeve’ packages that can curb the problems that arise. Similar in concept, the sleeves generally consist of closely woven fiberglass ‘tube’, or a sleeve, which is sometimes, covered with a high voltage resistant silicon material. What’s the added wire protection worth? According to the guys at Moroso the sleeve adds 850 volts of extra insulation, almost double that figure in cross wire ‘insurance’, and a profound resistance to heat created by those hot close-up all out race headers that usually get tucked into the most impossible places imaginable. The guys that design and manufacture these tightly fitted spaghetti pipes are to be admired for the work they do. Some of these applications are made to clear obstacles unimaginable, and they do it day in and day out.

Virtually all sleeves slide over the ignition wire. As expected, one end of the wire has to be sans terminal and boot, which are then installed after the sleeve and associated shrink sleeves are slipped over the wire. Existing wire sets can be protected via the sleeve material, but you’ll have to remove and install the wire terminals and associated boots. The various wire sleeves don’t fit tightly over the wire-especially at the boot. Given this fact, a system of sealing the sleeve to the boot is usually incorporated. Moroso and MSD offer a special ‘shrink sleeve’ to totally seal the wire sleeve to the respective dizzy and plug boots. The shrink material fits part way over the boot and extends part way over the sleeve, sealing the actual wire sleeve to the boot. In order to make the seal, the material is physically ‘shrunk’ via a heat gun or naked flame. (Care should be exercised when using one of those open flame throwers for obvious reasons, so try and find a heat gun somewhere for the task at hand).


If you are looking to running low times at any ¼ mile run or just out on a Saturday night you need to ignite the cables with the best possible distribution of current. There are a number of different rotors on the market place. We highly recommend a MSD alkyd rotor or an Accel equivalent. These rotors (with alkyd construction) have a higher resistance to carbon tracking than the OEM part or jobber components. Also, the MSD and Accel rotors have much larger arc ribs surrounding the rotor blade and adjacent to the rotor screw holes. Other features include riveted brass blades, which are longer than some emission-style replacements, stainless steel spring construction and large, riveted contact buttons. In addition, we advocate the installation of nylon rotor screws in Delco-type distributors.

These rotor set up’s prevent sparks from arcing across the rotor creating simple insurance that the distributor’s spark will be correctly routed while spinning around delivering spark at the precise moment. Accel and MSD offer superior quality, performance-oriented dizzy caps. Constructed of an alkyd material, the caps are very resistant to carbon tracking and offer improved performance over a conventional assembly.  Further to this, MSD manufactures a special set of components called “Power Towers” that snap into the cap assembly.  These parts convert the cap to a spark plug style of wire retention.  This enables you to use large diameter 8mm plug leads in a cap that is normally jam-packed with smaller 7mm plug leads.  To add to this, the Power Towers provide for a more secure method of retaining the plug leads. In a nutshell, they will not fall out at will.  Another relatively new cap concept is the MSD “Cap-A-Dapt” system.  What it provides is positive wire retention (again with spark plug-type clips), extra wide post spacing and three-piece construction, which facilitate rotor phasing. Always check out what’s new and then ‘see’ if it’s going to work for your particular application.

  • AC/DC

High powered ignition systems add loads on the battery as well as the charging system.  When you increase the demand upon the battery, it should be capable of handling the load. Batteries are rated by capacity. This is the amount of electrical current or amps a battery can supply for a specific amount of time.  The “generally accepted” rating system is the “Cold Cranking Amps” (CCA) method.

The CCA determines the battery’s capability of delivering current (amps) under cold conditions. This rating will generally range from 250 to 800 amps, but keep in mind that many (if not all) CCA ratings are rather optimistic. To combat the factory rating game, compare the “Reserve Capacity” (RC) ratings of the batteries in question.
This rating determines how long the ignition and other electrical components can be operated by the battery alone (without the generator functioning).  The RC rating is defined as the length of time (minutes) that a fully charged battery can deliver 25 amps of power at 80°F (maintaining 10.5 volts).

In other words, the higher the RC, the better the battery.  Thanks to the guys at MSD, there is a formula, which can be used to easily convert the RC rating into the “old” (you can read that as “useful”) battery AH rating number.  The AH figured determines the number of amps a battery can deliver for one hour.  The following is the MSD equation:

25 amps x reserve capacity (minutes) = AH

Using the above formula, if the battery reserve capacity is 30 minutes, the AH capacity is as follows:

25 amps x 30 = 12.5 AH

Translated, this simply means that the battery can supply 12.5 amps of power for one hour.  MSD states that ignition operating time decreases as engine rpm increases.  The smallest dedicated battery for use on a MSD 6- or 7- series ignition system is a 12 AH unit.  There’s no question that today’s crop of performance cars are becoming evermore sophisticated.  Large capacity electric fuel pumps, electric water pumps, electric fans, high powered sound systems, and even small things like gauge lamps and other hardware sap power and AH from the battery.

You have to consider and first determine how many amps (AH) each of the electrical “accessory” components continuously draws from the battery.  As an example, you might be surprised to find that one of the large “professional-style” billet fuel pumps can draw 8 AH or more.  Next, add up the total AH draw.  Using the following chart from MSD, you can then determine the total AH draw of a typical high powered ignition system at a specific engine rpm.

RPM (V8 engine)                       MSD 6 & 7 Series

Imagine that you have one of the big 8-amp electrical fuel pumps in your car.  In addition, you have an electric water pump drive that draws five amps on a continuous basis.  Your other accessories draw a total of five more amps.  Finally, you engine sees a maximum rpm ceiling of 8000.  The addition is rather easy:

- Electric fuel pump: 8 amps
- Electric water pump drive: 5 amps
- Other accessories: 5 amps
Ignition requirements: 8 amps
Total: 26 amps

Next, multiply the total electrical requirements by the duration of time it must operate without the alternator functioning (i.e.: sitting at the local burger joint listening to the stereo).  Multiply the amp requirements by the time:

26 amps x half-hour = 13 amps

Now, the final step in the equation starting the car.  If your car has an on-board starter (and virtually all street cards do), multiply the final amp requirement figure by a factor of three:

13 amps x 3 = 39 amps
The final 39-amp number is the size of the battery required for the application.  Using the very first equation, which showed how to convert RC to AH, you can then go shopping for a battery (or a pair of batteries).

Applied Pressure


Before you can truly begin any serious tuning, you have to be sure that the engine you are going to spend hours working on is in good sound condition. It makes no difference whether the engine you have is a week old or if it has 30,000 km on the clock. There is only one way to accomplish this tune up correctly and that’s with a leak down test process. Compression tests might suffice for a ‘run about’ sedan, but remember, we’re talking about an outright street race, hi-performance engine.  The main theme behind a “leaking down test” is to pump a given amount of air into a cylinder and then measure how much escapes via the piston rings and valves.  You might be surprised when you hear the excessive compressed air blowing by a worn and leaking exhaust valve or perhaps an equally worn out intake valve. (Exhaust valves wear faster than intake valves).

Similarly, air can blow by the piston and it’s rings and enters into the sump.  While performing the test, keep in mind that a good healthy engine can leak less than 5%, while a chronic leaker can exhibit numbers nearer the 50% mark.  Some of the best-sealed engines in the drag racing circles are found under the bonnets of NHRA stock racing cars.  Take the ‘leak down’ numbers into account and what should they be?   Less than 3% on some of the better engines.  Use your own judgment when it comes to a leak down test, the figures should be well noted from cylinder to cylinder. Be advised that an engine that leaks over 10% is asking to be stripped apart, closely examined and accurately measured up to see where it can be improved upon.  Compression testers are available from a number of sources (single-gauge models).  Aircraft supply shops are also a good source if you can’t easily locate a leak down tester at your corner speed shop. Whatever you do, make sure the one you purchase or use is of a good quality.


Compression tests are routine tune-up steps, but on a high-performance application, they should not take the place of a leak down test.  Essentially, the idea of a compression test is to obtain a reading that is close to that specified in a factory service manual.  Now, if you have bumped up the static compression ratio (high compression pistons), changed the camshaft profile i.e. (changed the lift and duration figures from the standard factory figures), then the compression test numbers you get will be totally meaningless to anyone and that leaves you with an educated guess.

There are two areas however where a compression test can be of some assistance.  Low readings usually indicate that the rings or valves require maintenance. This can be checked by squirting some lightweight oil into each spark plug hole. Crank the engine over with your bump switch for a few seconds to allow the oil to get in under the rings and into the places that will help seal off the rings as best as possible and then repeat the compression test.

If the compression readings are significantly higher, then you have a problem with the rings, piston or cylinder bore.  If no changes are evident, then the problem lies with the intake or exhaust valve or both.

A drastic drop in pressure in one (or more) cylinders can mean numerous things. The worst could be a cracked piston or broken rings and a broken ring land combination.  The most common problem is a blown cylinder head gasket.  If a pair of side-by-side cylinders show approximately the same compression readings (while other cylinders are good), this is a good sign that the head gasket is blown between the cylinders. This is by far the most common problem, especially if the engine has been bored out to the max, bringing the cylinder bores closer together, thus narrowing the deck surface between the two cylinders.

Many engine builders opt for the largest oversize slugs available and most often end up fitting foreign pistons to the build and end up machining off the top of the piston, pockets or dome – a common practice amongst engine builders in time, somewhere along the line.

But if you can afford the right equipment then you will find the correct hardware at your local speed shop.

Valve Head Gear


One area of the power search that most of the enthusiasts tend to forget about are the rocker arms and their geometry. These are probably the best lessons anyone can learn. In most cases you simply purchase a new set of rocker arms and bolt them onto your cylinder heads. Unfortunately, the trouble with that plan and particularly with small and Big Chevrolet, FoMoCo (Ford), Windsor, Cleveland and 385 Mopars and a few others is that the factory rocker arms can often be way out in the ratio department. Using a small block Chevy engine as an example, the OEM or (original equipment manufacturer), specs call for a rocker ratio of 1.5:1. This in simple terms means that the rocker arms will multiply the camshaft lobe lift by 1.5 times. If the lobe lift on a small block camshaft is .400 of an inch on both the intake and exhaust lobes, you can multiply it by the rocker ratio of 1.5 and the gross valve lift should be .600 of an inch.

Unfortunately, a stamped steel rocker ratio might only check out at 1.43:1 or even less. As a result, the gross valve lift works out to .572 inch. The valve train effectively has lost about 5% of the total lift. This only gets worse as the lobe lift numbers increase (the camshaft becomes more radical).

What’s the answer to the problem at hand? There are several solutions, but if you can’t afford a trick set of aftermarket roller rockers, the answers start hiding themselves from you-rapidly. The most inexpensive option is to check a box full of rockers until you find all 16 of them that have the highest effective rocker ratio and are that close to one another. In order to accomplish this, you will have to get into action and install a solid lifter on one rocker arm to zero lash. Next is to install your dial indicator to read off the valve stem side of the rocker arm. Turn the engine through one complete revolution with a power bar fitted with an extension and socket.

Compare this gross (zero lash) to your camshaft specs. You might be surprised to see that the numbers do not correspond. To verify the figures, either verify the number with your camshaft specifications car or check the lift at the lifter and multiply the number by your rocker ratio. This number is the theoretical gross valve lift. In many situations, a factory rocker arm will have a ratio that is significantly less than you would of imagined.

As mentioned previously, most small block rockers ‘check’ at between 1.4:1 and 1.47:1, very few attain the advertised number of 1.5:1 in the ratio department. To correct the problem you can either rummage through boxes of new rockers until a ‘perfect’ set is found, or install a set of aftermarket rockers. If you install aftermarket rockers, be absolutely sure to verify the ratio. Performance aftermarket parts aren’t perfect either.


Is there a difference in lash procedure between an engine equipped with stock rockers and one equipped with roller rockers? There’s none in terms of lash numbers, but there is one thing you have to remember. When lashing valves with OEM rockers, you can sometimes slide the feeler guage in at a at bit of an angle. This isn’t possible with a roller tip rocker. If your engine is equipped with roller rockers, be certain that you slide the feeler guage in a straight line between the rocker tip and the top of the valve.

In any case, the idea is to use a ‘go-no-go’ system, keeping it as smooth as possible. In other words, if the cam company calls for .024 inch lash, then a .024 inch feeler guage will fit, but a .025 won’t. After some practice with your particular combination you’ll get a real feel for the correct lash. Some guys like a ‘tight’ pull on the feeler guage, others don’t it’s just a matter of a personal likes or dislikes of styles.


When working with aluminium cylinder heads and/or aluminium cylinder blocks, cold lash numbers can vary greatly from the hot figures. So why does this happen? Simply because aluminium moves by expanding and contracting a great deal more than cast iron when heated up. Because of this you can understand why and how valve lash figures become decidedly different with ally combinations. Although it’s difficult to provide the hard and solid numbers for all camshaft and engine combo’s, Chevy offered this advice as a rule of thumb: ’Cold-lash all ally engines .010 inch tighter than the Hot-lash specs’.

Generally, you can use this as a good starting point and go on from there. Some ally head to iron block combinations are very close to an all iron engine in terms of cold lash. Others might be and well be anywhere from .005 inch to .010 inch tighter. Your best chance is to contact your guy who grinds camshafts for you and ask him for a specific cold-lash number for the particular combination you may have.


Eyeing the timing marks is always going to hinder any mechanic in a dark engine compartment and is never easy and to make matters worse, the timing marks become harder to read when unnatural or partial light is directed into the engine compartment. Even the addition of a degreed balancer or a timing tape can still make for some partial visual impairments. Most of the time the problem isn’t always the degreed markings on the balancer. Instead it’s the timing mark pointer that’s not accurately lining up and showing you the exact marking that can be exactly pin pointed. Trimming your standard pointer to a ‘V’ shape at the zero marking. The pointer is easier to read and eliminates confusion over the timing marks location.

In addition to this, aftermarket companies have designed bolt-on timing pointers with an adjustable pointer that can be easily set up to read ‘on-zero’ or from four degrees retard to 16 degrees advance. From this simple little addition of the adjustment, the pointer makes any timing checks easy and simple and also solves the timing-out, pointer problems that can sometimes plague engines and engine builders. Occasionally, the TDC mark on a factory harmonic balancer will be slightly out and so can the timing pointer. This is unacceptable to any standards and all adds up to an extremely inaccurate ignition timing numbers as well as valve lash figures that can be out.


A valve springs life is always critical to any performance street rod engine.  How can the valve’s spring life be considerably improved?  The first step is to pre-stress new valve springs prior to their installation. In other words, the springs are squeezed to the limit by a soft-jaw vise and compressed several times before installing the valve spring to its place. The idea isn’t to damage the spring in the vise but instead, the spring should be compressed just nicely, without pushing the spring into a coil-bind, but just enough to push it to the limit a few times but not over.  (Watch the spring carefully as you compress it together). Install the spring on the cylinder head and check the seat pressure. If the spring fails miserably, “tag” it or return to the selling dealer and install a new one. If you don’t have access to a soft-jawed vise something soft should be attached to either side of your vise jaws.  The cushion you have now added saves the valve springs from being scarred by the course steel lining jaws of the vice.

Here’s another method you can try for improving the life of the spring:

Inspect the inner spring and dampener carefully. You might find that some valve springs have added “flashing” on the spring ends.  (This is common on some types of dampers.)  If that’s the case with your set of springs, use a small die grinder and very carefully smooth over the odd burrs and high spots that are present. Similarly, some dampers have very sharp edges on the “flats.”  The life of the damper can be improved by gently de-burring and chamfering them, and it can be done in five minutes flat - if you have the right tools laid out and at your disposal.
It should be pointed out that damper failure is more common than we’d like to think (especially on high lift, radical profiled cams).

Occasionally, a damper will physically “unwind itself” and the lower portion of the assembly will work its way between two lower coils of the outer spring. Naturally, this stacks the spring into a coil bind. When this happens, all kinds of carnage can occur if you don’t nip the problem before this can happen. In most cases, selecting the correct length of damper will suffice, but if the problem hampers your application, you can solve it by shortening the damper a few mills. You might try this old racer trick (its been around for about 30 years or more).  Sand blast the damper after it has been de burred and the edges chamfered.


When the time comes to installing the valve springs on the cylinder heads, have a close look at the relationship between the inner spring and the damper to both the cylinder head seat and the valve spring retainer. Because of the many different types of designs manufactured in springs, valve retainers and spring seats, there might be some coil-bind at these locations, but no coil-bind on the outer spring. Have a close look as the engine is turned through a cycle (manually), don’t use the starter to spin the engine over, it will be too fast and your eye wont be able to follow the springs exact motion, not to mention what can be damaged along the way.

In the case of a poorly selected spring (or spring retainer), don’t be surprised if you see coil bind on the inner spring(s). If that’s the case, you will have to pull everything to pieces and install an inner valve spring that suits both the application and the spring retainer. This exercise must be executed clinically or you will end up with catastrophic complications that will set you back thousands of rands and sleepless nights.

While you’re at it, regularly check the springs with a seat pressure tester (inexpensive models such as this Moroso unit are readily available). These testers simply slip over the rocker arm.  Add a bit of muscle power and pull down on the tester. The number that appears on the beam scale (its like a beam torque wrench) will give you the spring seat pressure reading. If the seat pressure is down from the manufacturers specifications, you can bet your bottom dollar that the valve springs are tired and need to be replaced with new ones. Make it a practice to check the spring seat pressure every time the valves are lashed. The checking process adds about 15 minutes to the routine maintenance schedule and is always very rewarding once the above process is completed with dedicated accuracy.


Setting the valve lash on any solid lifter engine can get old in a hurry. Especially if you have to climb inside the car, tap the ignition switch and run around to the engine and check the balancer-timing tab location. Temporary remote starter leads are one answer, but who wants to burn their hands every time you hook it up?  The solution is a permanent bump starter switch.   Moroso and other companies offer state of the art waterproof switches that can be mounted somewhere in a convenient location.

Wiring the bump starter switch is simple. Wire one lead to the battery cable lead on the starter while the other lead is routed to the switched “Starter” or “S” terminal on the starter. When in operation, the ignition switch remains untouched. Simply press the bump starter and allow electrons to spin the engine over to the appropriate balancer location. When a “bump” starter switch is installed, it takes away most of the aggravation usually experienced, the down side is that you’ll still have to use a large half-inch drive bar to rotate the engine to the exact pointer location. In most cases, you can ‘bump’ the engine to a point, which is close enough to your mark, and then use the power bar to move the balancer and timing marks up to the exact position.  This procedure eliminates the brute force required to turn over the engine. The final grunt work can place the timing marks in the ideal position. Fitting a ‘bump’ switch will relieve you from some tedious pressure when getting down to the basics and for the effort of fitting one you will never look back.

Engine Tuning Tips

Here’s A Box Of Tricks
To Make Your Passes Quicker!

Hi Performance Street Rod skills aren’t hard to learn and in many situations they’re almost simple. Unfortunately, its tough to tune a car with a set of braced up Weber carbs if you don’t know how to set up a single carb. By the same token, it’s impossible to cool down a high compression, radical big block engine if you’re having trouble cooling down your mom’s old Malibu.

In sports they call it ‘basic skills’, if you don’t have the basics down pat, you cant run the quick E.T’s. To make it worse still, there aren’t any many weekend workshops or places you can go to learn the basic skills of ‘engine tweaking’ and modifying your muscle car to cut down those tenths. No, there’s no university to teach you the skills of engine building, engine tuning and hot rodding, but there is a substitute for the ‘school of hard knocks’.  Check out the following how-to guide, there’s a boxful of tips below to get you there quicker.