Squirt gun for the valve-and-cam set

A four-stroke motocross bike's biggest liability is not access to oxygen, but to fuel. You gotta give to get and fourstrokes don't get enough fuel at low rpm. When a Keihin FCR carburetor's slide is jerked open, more air reaches the cylinder head than gasoline. This mismatch produces a bog (sometimes referred to as a cough).

It can be fixed. The quick and easy way to stop your thumper from coughing is to inject more fuel into the intake stream at low rpm. The method of injection is the accelerator pump. The diaphragm-controlled pump mechanically squirts gas into the cylinder. Without the accelerator pump, the CRF would bog and momentarily hesitate until gas atomization caught up with the flow of air.

How much do you know about your bike's accelerator pump? Not much? Would you like to know more? A lot more? As always, the MXA wrecking crew is here to give you more information than you'll ever need.

Pumper carb: Because of lag in engine speed when the throttle is first turned, a four-stroke can't draw enough gas from the float bowl. The accelerator pump mechanically squirts gas into the engine.

 

 

 

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 PUMP?

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 carb works:

(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 situations.

(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 forcefully.

(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

 

 

 

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Last Updated 06/16/2004