the accelerator pump system consists of three main components: the pump
diaphragm
the pump cam and the pump nozzle
this is the carburetor system that is most responsible for having good
crisp off idle throttle response its purpose is to inject a certain amount of
fuel down the throttle bores
when the throttle is opened by accomplishing this purpose
it acts to smooth the transition between the idle and main circuits
so that no stumble hesitation or sluggishness will be evident during the
transition phase
differences in vehicle weight transmission 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 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
little fuel too late the first thing to check is the clearance between the
accelerator pump diaphragm
arm and the operating levers screw and spring assembly
start by making sure the accelerator pump arm is just touching
the bottom of the operating lever screw if their slack
hold the screw at the bottom and adjust the nut up top
until there's no slack but also so that the pump arm
isn't being pushed up now hold the throttle lever to the wide open throttle
position
you only a feeler gauge with the 1/15,000 blade
slip the blade between the pump arm and the adjustment screw bottom
before you do this check make sure the fast idle cam lever
is disengaged so that the throttle arm is fully seated
if there's proper clearance you'll see the pump are move down a bit
when the blade is inserted when you pull the blade out you'll see it move back up
a bit when it snaps back up to the screw bottom
it'll be a slightly snug fit and you'll feel a little drag when you move the blade back and
forth
the purpose for this clearance is to assure that the pump diaphragm is never
stretched to its maximum limit at wide open throttle
which could cause premature pump failure the amount of fuel that can be delivered
by one accelerator pump stroke is determined by the pumps capacity
and the profile of the pump cam the period of time that it will take for it is
predetermined amount of fuel to be delivered
is affected by the size of the pump nozzle
or squirter a larger pump nozzle will allow the fuel to be delivered much
sooner
than a smaller pump nozzle if you need more pump shots sooner
then a larger pump nozzle size is required
during acceleration tests if you notice that the car first hesitates
and then picks up it's a sure bet at the pump nozzle size should be increased
a backfire or lean condition on celebration also calls for
a step up in pump nozzle size conversely
if off idle acceleration does not feel crisp for clean
then the pump nozzle size may already be too large
in this case a smaller size is required 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
say numbers 35 through 40 will 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
such as vehicles with good power to weight ratios best acceleration
is achieved when the accelerator pump delivers lean
best power fuel to air ratio to the engine not when the maximum volume
of fuel supply when tuning a double pumper
an important point should be kept in mind 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 main nozzles in the venturi don't start by the time the pump
shot finishes
bogging will result Holley accelerator pump nozzles are stamped with the number
which indicates the drill pump hole size
for example a pump nozzle stamped with the 35
is drilled 1/35000 of an inch in diameter
pump nozzle sizes are available from numbers 25 to 52
you should note that whenever a number forty or larger accelerator pump nozzle
is installed
the hollow pump nozzle screw should also be used
this screw will allow more fuel to flow to the pump nozzle
which will make sure that the pump nozzle itself will be the only limiting
restriction
and not the threads of the mounting screw for the squirter
also be aware that underneath each squirter there's a check valve needle
during assembly if you turn the carburetor upside down while the squirter
is removed
this needle might fall out when changing the pump nozzle it's best to jump three
sizes at a time
for example if there is currently an offline hesitation with a number 28
pump nozzle
try a number 31 pump nozzle also
if you end up needing to use a thirty-seven or larger pump nozzle
then you should also use a 50 CC pump diaphragm assembly
when maximum pump capacity is desired a 50 CC accelerator pump conversion kit
is available under Holly part number: 20-11