Service manual

11
ACCELERATOR PUMP TUNING:
The accelerator pump’s purpose is to make up for the lag in fuel delivery to enable the engine speed to increase in response to
throttle opening. Differences in vehicle weight, transmissions, and rear axle ratios affect the amount of fuel and the delivery rate
that should be provided by the accelerator pump. This may necessitate the customizing of your accelerator pump to your
vehicle and its use.
NOTE: The old saying, “if a little is good, a lot is better”, does not apply to the proper tuning of the accelerator pump.
Your car’s performance can be just as bad if it receives “too much fuel too soon” as if it receives “too little fuel
too late.”
Two factors that affect the accelerator pump’s delivery are the pump cam and the pump shooter (discharge nozzle). The pump
cam determines the total volume of fuel and affects delivery rate; the pump shooter affects delivery rate and helps determine the
duration of the shot.
In general, the #1 location on the pump cams provides a moderate initial delivery and has a greater final delivered volume. The
#2 locations on the pump cams provide a greater initial delivery and have a lesser total delivered volume. The pump shooters
have a number stamped on their casting, which designates the shooter size in thousandths of an inch, i.e., a #25 shooter has a
.025” discharge orifice. The smaller diameter nozzles lengthen the pump shot duration and are used with heavier vehicles or
with vehicles equipped with lower numerical rear axle ratios. Larger diameter nozzles (.035 - .037) shorten the pump shot
duration, but deliver a greater initial volume of fuel. These sizes should be used on applications where engine speed will
increase rapidly (vehicles with good power-to-weight ratios). Best acceleration is achieved when the accelerator pump delivers
the lean, best-power, air/fuel ratio to the engine; not when the maximum volume of fuel is supplied.
An important point should be kept in mind when tuning a Double Pumper™; the secondary accelerator pump must supply fuel
for a sufficient time so that the secondary main nozzles can “start up” and deliver fuel to the engine after the secondary throttle
plates are opened. If the nozzles do not start by the time the pump shot expires, bogging will result. To apply the information
above, follow these steps for tuning the accelerator pump.
Figure 13 Figure 14
1. Change pump shooters until the smallest diameter nozzle that provides the crispest response is found.
Figure 15
2. Then change the pump cams and locations until the right cam is found that provides even more response. (Holley® offers a
pump cam tuning kit part P/N 20-12)
3. Finally, change the pump shooter once again, until the crisp response is maximized.
NOTE: If a nozzle size is desired that seems “in between” the nozzle sizes provided, then the nozzle can be drilled to
the desired size by using a wire drill held in a pin vise.