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Performance engines are like people –each one is a little bit different. So with a Holley 750 cfm 0-3310 carburetor or one of the hundreds of other models, these carburetors are intentionally built for a generic application. Once the carburetor is bolted to your engine, it will probably work extremely well. Or, it may need some additional tuning to allow it to perform its best. This discussion is aimed at the accelerator pump circuit. This story will be aimed primarily at street-driven engines and their requirements although all this information can also be applied across many different engine operating regimes.
All carburetors use some type of accelerator pump circuit. This is necessary to compensate for a very quick opening of the throttle. Carburetors rely on air velocity, through the carburetor to operate the main metering circuit. This circuit uses air speed through the boosters to create a low pressure that will push fuel from the float bowl into the engine. This occurs because atmospheric pressure in the float bowl pushes fuel toward the low pressure area in the boosters.
The main metering circuit does not deliver fuel on an instantaneous basis. If the throttle is slammed open, it takes a moment or two for the main metering circuit to respond to this change in throttle position. In these moments, the engine will experience a lean air-fuel ratio condition that will create a hesitation or even an intake backfire. The accelerator pump circuit addresses this condition by adding the additional fuel to prevent that momentary lean air-fuel ratio..
Let’s first describe exactly how the accelerator pump circuit works and then we can get into tuning. Fuel in the bowl feeds the accelerator pump reservoir through a one-way check valve. Inside the accelerator pump reservoir is a small diaphragm and return spring. The diaphragm is acted on by a series of levers initiated by a nylon eccentric located on the primary throttle linkage.
This lower view reveals how the linkage operates. The nylon cam (1) on the primary linkage pushes upward on the follower, transferring that movement through the linkage (2) to the pump arm (3) that pushes upward on the diaphragm located inside the 30cc housing (4).
This is the float bowl side of the primary metering block. Fuel from the accelerator pump housing enters the lower right side of the metering block (arrow) and travels diagonally upward to meet the vertical accelerator pump nozzle passage.
When the throttle linkage is opened, the cam pushes down on the lever which is connected to the accelerator pump diaphragm. This action pushes upward on the diaphragm which forces fuel up through a diagonal passage in the primary metering block, connecting to a vertical passage located in the primary venturi area. Inside this vertical passage is a check valve that retains fuel above the float level so that there is fuel present as soon as the circuit is activated. This passage is topped with a small, round nozzle stamped with its output size and retained with a hollow screw that allows fuel to pass into the nozzle.
The nozzle (or squirter) features two calibrated holes that inject fuel into each venturi. The fuel is intended to compensate for the short amount of time it takes for inlet air velocity to reach the level that activates the main metering circuit. Because timing of the fuel is crucial, the fuel needs to be injected as soon as the throttle begins to open. At very slow throttle openings, the accelerator pump delivers very little fuel because the off-idle transfer circuit can accommodate this condition. The accelerator pump circuit's primary task is to compensate for rapid throttle transitions from a near-idle, especially under high engine load as with a drag-strip style start.
We should also take this time to mention that all Holley carburetors use an accelerator pump on the primary side, but vacuum secondary four-barrel carburetors do not employ a pump on the secondary side. "Double pumper" or mechanical secondary carbs employ an accelerator pump on both the primary and secondary sides. An accelerator pump is not necessary on vacuum secondary carbs since secondary opening is actuated only when sufficient air velocity occurs on the primary side. This ensures there is sufficient velocity to instantly trigger fuel flow from the secondary boosters. If a vacuum secondary carb hesitates when the secondaries open, the secondary diaphragm spring is too light, opening the secondaries too soon.
Vacuum secondary-actuated four barrel Holleys (left) do not require an accelerator pump circuit on the secondary side. Mechanical secondary Holley carbs (right) do require a secondary accelerator pump circuit since the secondaries can open even at very low engine speeds.
All 4150 and 4160 style Holley carburetors use the standard 30cc pump reservoir. This capacity rating is not the amount of fuel delivered per full stroke of the accelerator pump system but instead is the volume after 10 strokes. This is the total reservoir capacity. Depending upon the pump cam, the volume delivered after 10 strokes is usually less than 30cc. As an example, the white accelerator pump cam will deliver less than 20 cc of fuel after 10 strokes. The nozzle or squirter size has no bearing on the volume delivered. A smaller nozzle like a 28 will merely create a longer duration shot of fuel versus a larger nozzle size.
You may also have seen references to a larger, 50cc pump that can be added. For even aggressive, high-horsepower applications, it would be extremely rare for a tuner to find a situation where a street car was in need of the extra volume of the 50cc pump on a 4150 style Holley carb. These larger reservoirs are found on much larger, 4500-series Dominator carburetors.
One tuning point that will be stressed several times throughout this story is that the ideal accelerator pump tune is the least amount of fuel required to create optimal acceleration. When a hesitation occurs, the oft-used solution is to add more fuel with a larger nozzle or with more lift from a taller pump cam. As we mentioned, this shot of fuel is only intended to prevent a lean hesitation. It deserves mention that engine hesitation or bog under heavy throttle can also be caused by excess accelerator pump fuel.
Armed with the basics of how the circuit works, we can now approach the tuning aspects. Among the two most commonly used accelerator pump components are the accelerator pump squirter or nozzle and the pump cam. But before we get into those pieces, it’s always important to make sure the current system is working properly. The most common cause of an annoying off-idle hesitation on Holley carbs is when the primary bowl has been removed and re-installed. It is very easy to replace the fuel bowl in a slightly different position that can create additional clearance between the accelerator pump cam linkage and the pump. This will delay the pump shot into the engine and can cause a hesitation.
Holley’s recommendation for setting the pump linkage refers to ensuring that the linkage does not bottom out, creating a bind that could bend the linkage. Holley recommends 0.015-inch of clearance between the lever arm and the pump linkage at max lift (WOT). This spec is often misinterpreted to mean there should be this clearance with the accelerator pump linkage at rest. Too much clearance at curb idle will create a slight hesitation. Adjust the accelerator pump linkage at its curb idle position so that as soon as the throttle is moved, this imparts movement in the accelerator pump linkage. This can be easily adjusted with the spring-loaded bolt and nut arrangement on the pump linkage.
Often, a tuner can fine-tune a pump shot by moving the accelerator pump cam mount from the No. 1 to the No. 2 location. The Number 2 position delays hitting the peak cam lift on the eccentric – adding lift later in throttle position. Holley recommends the No. 1 position for idle speeds below 900 rpm and suggests moving to the No. 2 position for engines that idle above 900 to 1,000 rpm.
Here’s a simple tip that can save tons of effort. Make sure the accelerator pump linkage moves the arm to the diaphragm as soon as the throttle is moved. Clearance at the lever will create a hesitation. Adjust the spring-loaded bolt and nut until the clearance is eliminated. This should require only a slight adjustment to minimize the clearance. Do not preload the lever. Excess clearance at idle is a very common occurrence that takes a minute to repair and when adjusted drastically improves drivability.
If the engine persists with a slight hesitation off idle or the engine stumbles when the throttle is rapidly opened from curb idle, then the accelerator pump circuit is a great place to begin tuning efforts. As previously mentioned, a hesitation on hard acceleration can occur as a result of too much fuel (overly rich) as well as from the more common situation of too little fuel (too lean).
Let’s assume that we have a small-block engine with an aggressive camshaft, moderate compression, a single plane intake manifold and a one-inch spacer added between the carburetor and the intake. The larger plenum volume created by the spacer can require more fuel from the accelerator pump circuit. Let’s put a 750 cfm Holley HP carburetor on this engine (PN 82751). This is a mechanical secondary carburetor with accelerator pumps on both the primary and secondary sides- often referred to as a double pumper.
The stock primary accelerator pump nozzle size on this carburetor is a 0.031-inch while the secondary side is slightly smaller at 0.028-inch. A simple cure might be to increase the size of the primary accelerator pump to perhaps 0.035-inch and evaluate the change in engine performance. Another idea might be to increase both the primary and secondary sizes rather than adding all the additional fuel to just the primary side.
In a pinch when a larger accelerator pump nozzle is not available, the holes can be drilled with a pin vise and small drill bit to a larger size. Conversely, when a much smaller nozzle size is desired, we’ve drilled these passages and then filled the opening with epoxy and then re-drilled the passage to a smaller size.
Most tuners only think in terms of initial volume when considering changes to the accelerator pump circuit. It’s worth noting that adding pump cam lift also extends the duration of the added fuel shot. This may be getting into tuning minutia, but it is worth considering. Again, minimizing the fuel needed to maximize acceleration is this circuit’s only purpose. On a road race or autocross application where the throttle is moving almost constantly, this could become an important consideration to prevent pumping too much fuel into the engine. In this situation, less fuel could be a benefit. Repositioning the pump cam with the small screw attachment to the Number 2 location will delay pump cam operation and may improve this situation.
Holley sells an accelerator pump cam assortment kit with 8 cams with seven that offer different lift shapes to custom-trim the accelerator pump circuit.
Holley offers a pump cam assortment kit with a total of eight different cams – seven that are different from the stock white cam used on the primary side of most Holley carburetors. These cams offer options relative to the amount of pump lift per degree of throttle opening. This added lift equates to additional fuel delivered by a given throttle opening. This creates a situation where a hesitation could be mitigated with more fuel delivered even with no change to the nozzle diameter.
This discussion is intended to offer the basic parameters a Holley carburetor tuner can use to make some minor adjustments to solve minor hesitation issues. The Holley four-barrel carburetor has been around for decades and is highly responsive to even slight calibration changes. The best tuning approach is to make a minor change and drive the car to evaluate the results. Perform one change at a time and record your results. You’ll find the Holley is a very easy carburetor to adjust that you can become proficient in almost no time.
If a carburetor is dormant long enough, the check valve under the accelerator pump nozzle can stick, preventing fuel from exiting the nozzle. The fix is simply to remove the nozzle and dislodge the check valve with a small screwdriver. Then re-install the nozzle and test the circuit. Do not attempt to test with the nozzle removed as the pump shot will launch the check valve somewhere you will never find it! Worse, it may drop down inside the intake manifold where it could cause engine damage.
The stock replacement diaphragm works extremely well in most cases, but for older carburetors that are only occasionally used and then left unattended for months, we’ve seen the black diaphragm become brittle. The easy repair is Holley’s synthetic green pump diaphragm designed to stay pliable.
Carburetors with choke housings make changing the primary accelerator pump nozzle a bit more challenging. We’ve found loosening the screw and using needle-nose pliers will allow easier removal of the squirter.
Note: If you are unsure of the color of cam, or it has faded from use, each cam is also numbered.