Fueling the Carb
- THE PUMPER
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.
- CARBURETTOR FLOATING
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.
- JETTING THE SET
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.
- STARVATION TIMES
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.
- ENRICHING EDUCATION
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.
- PUMP ACCELERATOR
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.
- SHOOTING THE FUEL
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.
- BUMP CAMS
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.
- THE BALANCER
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.
