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By: Jason Giacchino
Email: offthepegs @ atvsource.com

April 2008 - Off The Pegs

Between The Carb & Fuel Injector

Edelbrock Carb
Edelbrock Carb

You donít have to be an industry insider to realize that of late much of the industry hype centers on fuel injection in the ATV application. Never mind the fact that cars (both race and street) have waved goodbye to the primitive gravity-driven carburetor long ago, for our unique little market, computer-controlled fuel delivery via pressurized lines is still in its infancy. However, with all of the attention on fuel injection, its little brother often gets left in the proverbial dust.  I recall my trusty 1992 TRX250X warning sticker that advised never to open the throttle before starting as the vehicle was equipped with an accelerator pump. The warning existed to prevent ham-fisted riders from flooding out their ATV by filling the carb with excessive fuel.  So, the question then becomes, exactly what is an accelerator pump, how is it different from fuel injection, and does it offer any gains in performance?

Whoa, one question at a time, please. Letís begin by taking a look at the basics. Boiled down to its simplest terms, an accelerator pump is a short-term remedy to an age-old dilemma. Say what? Letís remember that carburetors require a vacuum (created by the engine) to pull fuel through the jetting. The accelerator pump is a mechanical rubber-tipped piston that richens up the mixture to get things cooking until the whirling piston creates a vacuum strong enough to keep the fuel flowing on its own. Sound complicated? It shouldnít. The basic idea works something like this:

An additional jet (called a leak jet) provides a very small amount of fuel to the pump.  When you tap the throttle, this rod-driven plunger pushes a small supply of fuel into the intake track of the engine. Kicking (or pushing the little button) ignites this fuel, which, of course, begins the piston motion that creates the vacuum that replaces the need for the pump. The core idea of the accelerator pump is to provide a more stable starting procedure.

The next question is more self-explanatory. Unlike a true fuel injector, the accelerator pump is a mechanical (cam driven) extension to a typical carburetor. There is no need for a computer, battery, fuel pump, or pressurized system for the accelerator to do its job. But, at the same time remember that the pump isnít intended to keep on functioning once the engine is up and running. It is merely a device that targets hard starting (especially common on four-strokes).

As far as performance boosts go, no sir, this isnít the type of aftermarket accessory you should be looking into if a gain in horsepower or acceleration is what you seek. The principles governing the accelerator pump are founded on overcoming the physical shortcomings of a system, which relies upon itself to produce suction. This refers to those first few moments of starting when the motor coughs then stalls because it cannot draw fuel through the jets fast enough to satisfy its own thirst. Once running properly, the accelerator pumpís job is done (which it confirms once you blip your throttle to rev up the engine). So then, does this mean an improperly tuned or broken pump wonít affect performance either? Sadly, no, it doesnít. While the pumpís work is done when the system is working perfectly, there are situations when a malfunctioning pump can wreck havoc on your machineís performance by keeping that rich mixture when you are already warmed up and ready to go.

Fortunately, the system is mechanical (as mentioned above) and while true fuel injection systems literally require a fuel mapping computer to fine tune, the accelerator can be adjusted, tweaked, and tuned with some standard tools and a little bit of patience. There are variances in the wide variety of pumps on the market, but the best basic advice is usually to disconnect the accelerator pump linkage altogether and to fine tune the carb just as you would a model not equipped with a pump at all. We typically wire up the linkage so that it is all the way open. Then adjust the fuel screw and pilot jet as if the pump didnít even exist. Once the quad runs crisp with the pumpís linkage wide open, release it and punch the throttle all the way open. If the spring-loaded actuator starts to move before youíve hit the throttle stop, thatís the problem. Diaphragms are sold with various stopper heights for this exact reason. Remember that while waiting for a new diaphragm to come in at the local dealer, you can still ride your quad with the pumpís linkage wired wide open (in which case it would trick the carb into believing that it doesnít have a pump).

The bottom line is? Accelerator pumps are handy devices in this day and age of the high performance four-stroke engine. As electric start begins replacing kick-starters on even most race machines, the benefits arenít quite as major as they would have been in the kick-only days.  However, with a little care they are easily maintained and make firing up on those quickly approaching cool spring mornings much less of a hassle.


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