pumper carbs

new guy post... I'm seeing many references to "pumper carbs", what's the difference from a std carb, and does it affect how you would uncork (jet settings, etc)?


A pumper carb whether for a car, truck, motorcycle, etc, has an additional fuel circuit called an accelerator pump. This pump is manually linked to the throttle to provide an instant shot of fuel as the accelerator pedal is depressed or the throttle is twisted as opposed to all the the fuel being drawn in by the venturi effect. Most performance cars back in the 70's used pumper carb to minimize the off idle bog when the throttle was suddenly fully opened from a low engine RPM and the same thing is true for our 4 stroke big bore bikes. At low engine RPM, the manifold vacuum is high due to the throttle slide position and very little fuel is drawn in. When the throttle is suddenly fully opened, the manifold vacuum immediately drops and the engine momentarily runs lean until enough fuel can be drawn in. Some folks refer to this as vacuum lag. Because the accelerator pump doesn't work off of vacuum, it provides an instant shot of the fuel to alleviate that lean spot. Most performance 4 strokes have pumper type carbs for quicker off idle throttle response such as the CRF450, all the YZF's, KTM's, etc. An aftermarket carb with an accelerator pump can be used on our XR650R to get that extra bit of performance that's missing from our bikes, but like anything, some people find value in it and others don't. There's three main aftermarket pumper carbs for our bikes, which are the Mikuni TM40, Keihin FCR & the Edelbrock QwikSilver. They each have their strengths and weaknesses. The Keihin FCR's come stock on the CRF450R, YZF's, KTM's, etc and is a great carb, but it can also be the most difficult to tune and its also the most expensive. The TM40 & QwikSilver are more popular with our bikes because of cost, availability, support, easier tuning and they're also sold as complete bolt on kits so you don't have to piece anything together yourself. All these carbs have been discussed to death here, but you can find all kinds of opinions on each carb by doing searching the Honda forums here on TT.

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