Monday, October 20, 2014

Does Increased Displacement Require Leaner Carbeurator Jetting?

ASK KEVIN: Does Increased Displacement Require Leaner Jetting?


Mikuni carburetor product image
Question: Recently, I came across information which implied that when increasing displacement—such as installing a big-bore kit on a bike engine—if the carburetors stay the same throat diameter, the jetting might need to be leaner than that used with the previous smaller displacement. This seems counterintuitive. A larger displacement cylinder would require both more air and more fuel, so why would leaner jetting be needed?
Robb Millet
Milford, MA
Answer: I learned about this effect from the different cylinder sizes on three different Kawasaki two-stroke racers I worked with, all of which used the same-size 35mm carburetor. The 167cc cylinder of the H1-R 500 triple needed a 310-330 main jet, yet on the 250cc cylinder of the H2-R 750 triple, the same carb needed a 260-270 main jet. And on the 350cc cylinder of the F5 race engine, the main jet size went down slightly again, to 260.
Why? On a constant-size carburetor, the smaller the cylinder, the weaker the intake suction signal, and the smaller the fuel flow. Therefore, to give the 167cc cylinder the fuel it needs, a larger main jet must be used than for the more strongly pulsing 250cc cylinder. Flow through the carburetor is not steady and continuous, but is a series of pulses.
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