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The first thing to do when working on your jetting is to make sure all of these parts are working and correctly adjusted. 1/4 throttleĦ- Needle Jet - passage to which the jet needle residesħ- Jet Needle - regulates the flow of the main jet as the throttle is opened, 1/4 to 3/4 throttle.(raise the needle/drop the clip to richen, drop the needle to lean)Ĩ- Air screw - adjusts the amount of air to the idle circuit to 1/4 throttle(in=less air/richer, out=more air/leaner)ĩ- Throttle stop, or idle screw - adjusts the idle speed Here is a list of the parts involved in jetting a bike and short explanation of the task of each part.ġ- Fuel bowl - holds the proper amount of fuel to feed the jetsĢ- Shut-off needle - shuts the fuel off to the bowl when the proper level is reachedģ- Floats - Floats up to force shut-off-needle to closed position at proper level.Ĥ- Main Jet - provides proper fuel at high rpm, above 1/2 throttleĥ- Pilot/Slow Jet - provides proper fuel at idle to approx. The carburetor on most 2-stroke dirt bikes consist of a few more parts than your average carb. When you change a jet, you are simply giving the fuel a smaller or a bigger hole to travel through, thus changing the amount of fuel that is mixed with the air. Most require this screw, and a jet change. Some, very few, carbs only require adjustments to some sort of screw to obtain the correct mixture. The mixture screw is able to adjust the air in small amounts in order to fine tune the mixture usually at idle. fuel is sucked through this main jet, and mixed with the incoming air that is creating the vacuum or low pressure effect in the throat of the carb. Usually somewhere in the bottom of the bowl resides the main jet. The fuel enters the bowl from the tank through a small passage until the floats float high enough in the bowl to force the fuel shut-off needle into the passage and shut the fuel off to the bowl.
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The parts of the carburetor always include a main jet, fuel bowl, fuel shut-off needle, floats, some sort of mixture screw, and an idle adjustment. Although this ideal mixture is impossible to reach at all times, the different functioning parts of a carburetor work at all engine speeds and loads to try to create this mixture to the engine for a proper burn. The purpose of the carburetor is to provide the combustion chamber with the proper ratio of fuel to air or the" Stoich Mixture ".
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and what effect jetting has on a carburetor. I won't try to do a comprehensive explanation, just enough to understand why jetting is so important. The following was done on a 2001 CR250 Dirt Bikeįirst, a VERY brief explanation of how a carburetor works might be helpful for some.