The diaphragm carbs used on most kart engines have been adapted from power tools such as chainsaws. The usual mixture needle settings for power tools give a relatively constant fuel mixture curve throughout the rpm range from idle to peak rpm. This provides a stable idle, smooth part throttle operation and a limit to peak rpm.

In kart racing, the required fuel mixture curve is very different. Full throttle performance is most important and idle and part throttle performance are secondary. The best full throttle performance requires the engine to provide maximum torque at low rpm’s when accelerating off a turn and a high as possible rpm on the straights. This is accomplished with a rich mixture at low rpm’s and a leaner high rpm mixture.

 So, for kart racing, the low speed needle is set much richer and the high speed needle is set much leaner than the normal power tool application. These settings usually compromise a stable idle and require that the throttle be “blipped” to keep the engine running during pace laps. See the instructions for each engine for specific carb mixture needle settings.

Another adjustment in diaphragm carburetors is the pop-off pressure. This is the amount of pressure required to cause flow from the fuel pump side of the carb to the metering chamber. The pop off pressure is determined by a spring and lever arm arrangement. The pop off pressure may be adjusted by varying the length and tension of the spring. In general the higher the pop-off pressure, the leaner the fuel mixture at low rpm and conversely, the lower the pop-off pressure, the richer the fuel mixture at low rpm.

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