To understand why you need an accelerator pump, you need to understand the physics of liquids and vapors. The point at which a liquid turns to a vapor is its boiling point, and this is dependent on pressure and temperature. An increase in temperature or a decrease in pressure causes a liquid to boil sooner. As you know, fuels are highly volatile with certain amounts evaporating very easily.
This Tech Tip is From the Full Book, DAVID VIZARD’S HOW TO SUPER TUNE AND MODIFY HOLLEY CARBURETORS. For a comprehensive guide on this entire subject you can visit this link:
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For a well-tuned street engine, you may see 11 to 15 inches of manifold vacuum. Under the sort of running temperatures existing in such an engine at idle, just about all the fuel entering through the carb’s idle system evaporates almost the instant it goes by the throttle butterflies. This is good for a smooth idle because it ensures ease of ignition and nearly perfect distribution.
However, opening the throttle at even a moderate rate, momentarily causes the manifold vacuum to drop substantially. When this happens, fuel that is in vapor form within the intake system almost instantly condenses onto the walls of the induction system. This leaves the air within the intake lacking fuel. This lean out is so severe that it is virtually impossible to ignite even when compressed in a hot combustion chamber. The fix is to inject fuel to “fill in” this lean spot until an appropriate fuel circuit reestablishes the correct steady-state mixture ratio. The circuit that takes care of this transient situation is the accelerator pump.
Accelerator Pump Circuit
The accelerator pump circuit is comprised of a pump situated below the fuel bowl, a throttle shaft mounted metering cam, an actuating arm that transmits the motion from the cam to the pump diaphragm, and calibrated pump jets situated just above the venturis. As the throttle is opened, the fuel metering cam operates the pump lever arm. This in turn pushes fuel through the metering block to the pump jets and into the engine. The only time the accelerator pump system should supply fuel is during a rapid opening of the throttle. If the throttle is opened slowly, the pump system should not supply extra fuel to the engine.
Steps need to be taken to prevent what is commonly described as pump jet pull-over. This occurs when the airspeed through the carb causes sufficient depression in the vicinity of the pump jets, which activates them and typically delivers unwanted fuel. This fuel is in addition to that supplied by the main and PVRC jets. Although this condition of pull-over is sometimes used as a tuning aid on 2-barrel race carbs, it is generally not useful for a 4-barrel installation.
The accelerator pump system needs to counter the following two situations. So as not to discharge fuel through the squirters when the throttle is opened slowly, in some systems there is a controlled bleed from the pump back into the fuel bowl. Older carbs have a check valve located within the pump. When the throttle opening rates exceed a certain amount, the check valve closes and the fuel is redirected solely to the squirters. Also worthy of note is that some aftermarket Holley specialists drill a small bleed-back hole through the pump roof/float bowl floor. This hole needs to be small (about 0.012 inch), but it can be used as a tuning aid for the pump system in much the same way as Weber pump jet float bowl bleed-backs.
To minimize high-speed fuel pullover, there is a pull-over check valve situated just below the squirters. The intent here is that the weight of the stainless-steel conical-tipped needle closes the valve to the effects of any depression at the squirter discharge point, but is easily lifted off its seat when the pump comes into action. On carbs with vacuum secondaries, the accelerator pump system is typically installed on the primary side only. That said, I have used a big-CFM vacuum secondary carb with squirters for both primaries and secondaries. But that is more an exception than a rule. A mechanical secondary carb has a complete accelerator pump system for both ends of the carb. As far as tuning the accelerator pump system goes, it is all a question of injecting just enough fuel to compensate for the vapor drop-out and the sudden increase in airflow. The tuning of the accelerator pump system is essentially straightforward so long as you know about the available hardware and what it does.
Accelerator Pump Components
The stock accelerator pump on a 4150-style carb has a capacity of 30 cc for ten strokes of the pump. On Dominator-style carbs, the pump capacity is 50 cc for ten strokes. Although rarely necessary, the bigger pump can be installed on a 4150-style carb by using Holley’s conversion kit (PN 20-11). The kit includes a diaphragm and the appropriate linkage to facilitate the swap. The 30-cc pump is suitable for almost all applications. Just how much pump capacity is required is dependent on the size of the engine, cam duration, and volume of the intake manifold. Tall intakes with large ports and a spacer usually need a lot of pump shot, so they may be a candidate for a 50-cc pump on a 4150 carb. Again, if you order a carb spec’d for the job at hand, chances are that you have very little to do in terms of dialing in the accelerator pump system.
Pump volume is a measure of the amount of fuel available for one shot only. This volume obviously always needs to be somewhat bigger than the engine needs from the carb during the most demanding situation. The fuel cam on the throttle shaft dictates the amount of fuel and the rate it is injected into the engine. The size of the squirters mostly dictates the length of time of the injection phase. When the throttle is opened, the spring on the end of the fuel cam arm activates the pump arm. Rapid opening causes the spring to become compressed and injection continues until the spring has returned to its normal length. Three issues must be considered when tuning the accelerator pump system for optimal results: injection volume, injection rate, and injection duration. The cam on the throttle shaft combined with the pump’s size (30 or 50 cc) largely controls the volume injected during an idle to WOT situation. At part-throttle operation, the cam profile and the size of the squirter control the rate at which fuel is injected. Last, squirter size largely controls the duration.
Preliminary System Tune
After the engine is warmed up and you are satisfied that the idle speed is properly set up, your first step is setting the pump arm preload spring. To do this, tighten the nut/ bolt assembly that compresses the spring until there is a slight amount of clearance (0.001 to 0.005 inch) between the head of the adjusting bolt and the pump arm. Next, undo the adjusting nut until the bolt head is just in contact with the pump arm. At this point, turn the adjusting nut two flats to lightly preload the spring against the pump arm. With that done, it is set.
Now is the time to check those provisional settings. To do this give the engine a quick “blip” of the throttle. It should, without hesitation, respond in a nearly instantaneous fashion. If the engine stumbles when given a sharp blip of the throttle, increase the pump jet size until the hesitation is gone. If you are easing up to the size and no hesitation is experienced, you need to recognize what is needed when the engine is given throttle under load. It may need 0.002 to 0.004 inch bigger than required for a clean pull in neutral. This is especially true if the engine is equipped with a light flywheel or a small-diameter high-stall-speed converter. If a blip on the throttle shows black smoke in the exhaust, drop the squirter size by a couple of numbers at a time until the black smoke disappears. If the response is sharp and no black smoke is seen, you can go on the road or track to check out the pump action over the anticipated operating range.
Final System Tune
For the accelerator pump action to be optimal, you need to consider those three factors mentioned earlier: volume, rate, and duration. As a matter of reference heavy vehicles pulling from low RPM and gaining RPM relatively slowly require smaller pump jets and a longer duration. On the other hand, engines in light vehicles that pick up RPM fast need bigger pump jets and usually less duration. With all that in mind, let’s look at what may happen at the track and determine the possible fixes. Let’s assume that the pump action is not delivering sufficient fuel at some point during the injection phase. If this is the case, there are a couple possible problem symptoms. First, if when given full throttle from an idle (or near idle), the engine just bogs and never recovers, you can be assured that the accelerator pump is delivering substantially less than the engine wants. The cure is to increase the pump jet by at least 0.005 inch. Continue to increase it until no bog exists. In extreme cases, the entire capacity of a 30-cc pump may be completely used up. If so, the carb needs a 50-cc pump. However, this is only the case on some seriously high output engines. Typically a single 4150 on an engine up to about 700 hp is fine on 30-cc pumps.
Second, the vehicle initially launches then takes a dive for a moment before recovering. When this happens it is a sure sign of insufficient fuel due to a pump shot duration that is too short. Your first move is to use a slightly smaller set of squirters to see if there is any improvement. The smaller squirters extend the duration but reduce the rate. If the problem is a little better, continue to reduce squirter size until no more improvements are seen, but the problem still exists. If this is the case, you need to look at changing the pump actuating cams on the throttle shafts to ones that have a longer stroke.
At this point, refer to the pump cam characteristics in Figure 10.11 so you can make an educated choice of a cam. You need to primarily choose a cam that produces more stroke to deliver a greater quantity of fuel over a longer period. The rate at which the cam lifts is of no consequence if the throttle action is from idle or some relatively low RPM to wide open. Slamming to WOT rotates the cam instantly to its fullstroke position, so the slope on the cam does not figure into the equation. Experiment with cams giving increasingly greater stroke until the throttle response is sharp and fast.
During typical street driving, there are countless instances where the driver calls for a little more power. In doing so, the throttle moves a relatively small amount. This is when the shape of the cam and the location hole used to secure it to the throttle shaft comes into play.
All cams have two or three location holes that can be used to position and secure the cam to the throttle shaft. The hole 1 location puts the cam in its most advanced position while hole 2 retards the action by about 5 degrees of throttle shaft opening.
For most applications, where the idle speed screws are nearly closed and idle speeds are in the 550 to 750 range, securing the cam via hole 1 is best. When a big cam is used and idle speeds are in the range of 950 to 1,200 rpm, use of the hole 2 or 3 is usually more appropriate. Part of the reason for retarding the cam when a high idle speed exists is the throttle shaft has rotated more at the higher idle speed. It has, therefore, used up some of the pump cam’s lift, unless it was retarded some.
If the engine has been extensively tuned for the street and uses a short-duration high-lift camshaft and a healthy amount of compression, it should exhibit a high-idle vacuum. Under these conditions even a small opening of the throttle at low RPM causes the intake vacuum to drop sharply for an instant. This is when the most aggressive fuel dropout occurs, and for that, the setup for the pump action needs to come on fast.
If the engine has a relatively big cam and idle vacuum is low, the rate of change of idle vacuum is less and usually slower so a cam with an initially slower opening rate could well be what is needed. If the engine is a large-displacement high-output unit, a high-lift cam to deliver more fuel is needed. If it is a relatively small engine, less fuel (and consequently less cam lift) is required.
Written by Tommy Lee Byrd and Posted with Permission of CarTechBooks