Q: DO TWO-STROKE CARBURETORS HAVE ACCELERATOR PUMPS?
A: No. The intake velocity of a two-stroke is much lower
than that of a four-stroke, and this gives a twostroke's engine
vacuum more time to do its work. The slower moving air effectively
draws gas out of the jets and mixes it with air.
With a four-stroke, there is a lag in engine speed as the
throttle is opened. As a result, a four-stroke engine can't draw
enough gas up from the float bowl. That is where the accelerator
pump comes in. Four-strokes do not have adequate vacuum when the
throttle is cracked open. The fuel pump is a mechanically controlled
circuit that squirts gas into an intake stream that is too weak to
draw gas by itself.
Q: DOES A TWO-STROKE HAVE ANYTHING THAT RESEMBLES AN ACCELERATOR
A: Yes. Some two-strokes use a power jet carburetor to
provide an extra squirt of fuel. A power jet is an electronically
controlled jet circuit that flows additional premix when engine rpm
tells the black box to send more. A signal from the black box opens
a complementary circuit, via a solenoid, so that more fuel can be
injected into the combustion chamber. But, and this is the big
difference, power jets do not work at low rpm. Instead, they feed in
more gas when the engine is under its greatest load (typically from
8000 to 10,000 rpm).
Q: HOW DOES A PUMPER CARB WORK?
A: Carburetors with accelerator pumps are euphemistically
called "pumper carbs." The accelerator pump gets its fuel from a
special gas reservoir hidden inside the three-bolt cover plate off
to the right side of the Keihin FCR float bowl. Here is how a pumper
(1) A rubber diaphragm covers the accelerator pump reservoir.
When the throttle cables rotate the carb pulley, they activate the
accelerator pump's link lever.
(2) A rod at the end of the link pushes against the center rivet
of the accelerator pump's diaphragm.
(3) When the diaphragm is depressed, it creates pressure (much
like squeezing a balloon). This pressure squeezes gas out of the
accelerator pump reservoir.
(4) The fuel blasts out of the accelerator pump nozzle into the
throat of the carburetor. Underneath the nozzle is a one-way check
valve that prevents air from entering the circuit when the
accelerator pump isn't squirting.
In essence, turning the throttle turns a link, which compresses
the diaphragm, which squirts gas into the engine.
Q: CAN YOU ALTER THE WAY AN ACCELERATOR PUMP SPRAYS FUEL?
A: Yes. Under the plastic throttle pulley cover on the
carburetor there is a timing screw. The screw sets how soon the
accelerator pump squirts gas. Turning it in delays squirt action.
That means the slide will open further off idle before the
accelerator pump sprays gas. Turning the adjustment out shortens the
accelerator pump delay. To set the accelerator pump linkage, the
proper diameter adjustment rod is first placed under the slide. Then
the screw is positioned just to the moment when the accelerator pump
link lever stops having free play.
Q: WHAT HAPPENS IF YOU MISADJUST THE ACCELERATOR PUMP?
A: If the screw is set to a position that delays the
squirt of gas too long, the bike will bog because gas is not being
squirted until it's too late. Conversely, setting the screw too far
out partially depresses the pump, even when the throttle is off.
This reduces the amount of gas that fills the accelerator pump
reservoir and also causes the decelerator diaphragm to hit bottom
sooner. Either way, poor linkage adjustment leads to less squirt
duration and fuel volume.
Q: CAN YOU CHANGE THE DURATION OF THE SQUIRT?
A: Yes. Some bike makers carry up to four different
accelerator pump diaphragms. These different diaphragms allow a
mechanic to tune the duration of accelerator pump squirt.
How do the accessory diaphragms work? A center rivet in the
rubber diaphragm is what contacts the pump rod. Each diaphragm has a
different length nipple that limits how far it moves. A longer
nipple stops it from pushing as much gas, while a shorter nipple (or
no nipple) increases fuel flow. The longer and shorter nipples will
reduce and increase the duration of the accelerator pump's squirt.
Not all manufacturers list the different decelerator diaphragms
in the optional carburetor parts section of their manuals.
Diaphragm: Keihin carbs, specifically those found on the RM-Z
hi(-F CRF and YZ-F locate the diaphragm in the float bowl. You
gain access by removing three screws.
Rubber duckie: An accelerator
pump diaphragm is little more than a rubber bladder. When
compressed, it creates both suction and pressure.
Q: HAVE YOU EVER HEARD OF THE LEAK JET?
A: Probably not. Information on this bleed circuit and its
jet is missing from most owners' manuals. The leak jet is located
between the accelerator pump diaphragm and the venturi nozzle. When
the throttle is opened gradually, the gas being pumped by the
accelerator pump moves slowly. Since the leak jet is positioned
early in the circuit, it becomes the path of least resistance and
allows gas to flow back into the float bowl. Why does it reroute
fuel back to its starting point? If the accelerator pump squirted
too much gas at low throttle opening, the engine would go full rich.
This massive blast of fuel would cause the engine to bog (just like
it did when there was too little gas).
Q: CAN YOU TUNE AN ACCELERATOR PUMP WITH THE LEAK JET?
A: Yes. The larger the leak jet, the more gas goes back to
the float bowl (and the less gas goes out of the accelerator pump
nozzle). A large leak jet will reduce both squirt duration and
volume. The converse is also true; a smaller leak jet bleeds less
gas back to the float bowl and more of it into the engine.
Q: ARE ALL KEIHIN FCR CARBURETORS THE SAME?
A: No. Only Keihins that come stock on production bikes
have leak circuits. Aftermarket Keihin FCRs, like those sold by
Sudco, don't have the leak jet's supplementary circuit. Another
conundrum is that Keihin FCR carbs can change during different
production runs of the same bike. Thus, the same model of bike can
be sold with leak jets and diaphragms that are different from each
other. Compounding the confusion is the overall lack of tuning
information available to the public.
Q: ARE ACCELERATOR PUMPS PROPERLY TUNED FROM THE FACTORY?
A: Yes and no. The consensus among four-stroke tuners is
that the stock accelerator pump adjustment shoots too much gas for
too long a duration. Most tuners believe that the bike will hit
harder and run cleaner with less accelerator pump squirt. Mechanics
modify their accelerator pumps in several ways:
(1) The diaphragm can be changed.
(2) The size of the leak jet can be altered.
(3) The link lever can be modified to retard pump action beyond
what the adjustment screw allows.
(4) The throttle pulley housing can be drilled and tapped to
accept a spring-loaded adjuster screw. This screw is used to limit
how far the link lever moves and the duration of the accelerator
pump gas stream.
Q: HAVE YOU EVER HEARD OF A P-38?
A: No, not the WWII fighter plane. The P-38 is an
accessory accelerator pump cover produced by Factory R&D. The P-38
differs from the stock cover in these ways:
(1) The P-38 reservoir is 20 percent larger. It is not larger so
that the diaphragm pump can squirt more gas, but to keep the
accelerator pump primed during repetitious on/off throttle
(2) A timing pin in the center of the reservoir restricts
diaphragm movement. Factory R&D presets the length of the timing pin
and tunes the duration of the accelerator pump squirt to best
accommodate engine characteristics.
(3) The inlet gas circuit is larger and the outlet is smaller.
Gas fills the reservoir more easily, and the diaphragm squirts more
(4) It keeps the depressed diaphragm under greater pressure so it
can spring back harder. The reservoir refills with gas faster and
the accelerator pump resets quicker for the next shot.
Factory R&D's P-38 modified Keihin FCR shoots a more forceful
spray of gas for a shorter duration of time. They claim that the
stronger stream of gas atomizes more quickly, lessening the chance
of engine flooding. For info call (949) 206-0894.
Micky K: Keihin isn't the only source
of pumper carbs. Mikuni offers a model that is standard equipment on
the 2004 Husqvarna TC450.
Q: IS KEIHIN THE ONLY SOURCE OF PUMPER CARBS?
A: No. Mikuni has offered pumper carb kits for decades.
Their TM pumper carb kit is a popular Honda mod. Mikuni's TM
accelerator pump system can be tuned externally. Additionally, 2004
Husqvarna four-strokes come with the new accelerator-pump Mikuni TMR
carb. Limited information and tuning parts are available for it, and
many Husky owners end up switching to an aftermarket Keihin FCR.
Q: IS THE ACCELERATOR PUMP THE BE-ALL-END-ALL OF CARB DESIGN?
A: No. The end-all would be fuel injection. Fuel injection
is squirt jetting at every rpm range. For most riders, the
mechanically controlled accelerator pump is adequate for the job at
hand. If there is a fault with accelerator pumps, it's that they
can't tell whether the bike is running or not. If it isn't, a twist
of the throttle will fill the top-end with excess gas.
Reprinted from Motocross Action Magazine August 2004