Keihin North America, Inc. manufactures automotive systems, including air and fuel management systems, heating, ventilating and air conditioning systems, and electronic control systems. Keihin carburettors are common on Japanese motorcycles, and the company is affiliated with Honda. Jetting refers to the proper tuning and maintenance of a carburettor.
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All Keihin carburettors are pre-jetted, or tuned, before sale, so they are ready to be used as soon as they are fitted. The factory settings, though, are for stock motorcycles and watercraft. If you modify your engine in any way, for example by adding a racing exhaust system or pistons with higher compression, you may need to adjust your jetting settings.
According to motorsports racing engine expert Eric Gorr, fuel flow to a carburettor is controlled by four circuits. These are the pilot/slow jet, the spray bar/needle jet, the jet needle and the main jet. Three circuits control the air flow. These are the air screw, the throttle slide and the air jet. Jetting an engine's carburettor means tuning these circuits to ensure that gasoline delivery and engine performance are as efficient as possible. A perfectly tuned two-stroke carburettor and engine delivers an air-to-fuel ratio of 12.5-to-1.
Throttle Opening Positions
The circuits affect the carburettor jetting for different throttle opening positions. For closed to 1/8-open throttle, the air screw and pilot/slow jet affect the jetting. For a throttle set between 1/8 and 1/4 open, it is the air screw, pilot/slow jet and throttle slide. Opening the throttle more, between 1/4 and 1/2 throttle, means that the throttle slide and jet needle are instrumental. The jet needle, spray bar/needle jet, main jet and air jet are all employed when the throttle is between 1/2 to fully open.
To check or set the float height, ensure that the float is resting on the spring-loaded float valve pin, not depressing it. You may need to tilt the carburettor so that the float tab touches the valve pin. If necessary, to achieve the correct height, carefully bend the metal tab on the float arm. The correct float height for all FCR carburettors is 9 millimetres (0.354 inches). The correct float height for all CR carburettors and PE 24 - 38 carburettors is 14 millimetres (0.551 inches). The correct float height for PWK 28 carburettors is 19 millimetres (0.748 inches). For PWK 35 - 39 carburettors and all PJ carburettors, the correct float height is 16 millimetres (0.629 inches). The correct float height for PWM 38 carburettors is 6.5 millimetres (0.255 inches). For PE 30 - 34 carburettors, the correct float height is 20 millimetres (0.787 inches) and for PE 36 - 38 carburettors, the figure is 22.5 millimetres (0.885 inches).
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