You may have heard the term "jetting" before and wondered what it meant. Jetting is the act of adjusting the separate fuel circuits in a carburetor to optimize the fuel/air mixture going into the engine. To the untrained person jetting can come across as being very complicated and some people fear the process. Once you understand which circuits control what part of the throttle cycle, you will learn that jetting isn't at all that difficult. The following will describe what each circuit does and how to adjust that circuit for optimum performance. This assumes that you have a Keihin FCR type carburetor (the information translates to other carburetors as well) which is found on almost every modern high performance four stroke ATV engine.
Fuel screw: Adjusts the "hang up" of the engine when you go from wide open to idle quickly. If your engine doesn't smoothly idle back down or "hangs up" when you let off the throttle you need to adjust the fuel screw out (richer) to clean this up. If your engine idles down and then dies you need to turn the screw in (leaner) to allow less gas into the engine. Play around with the screw until your engine idles down smoothly.
Pilot jet: Controls 0 to 1/4 throttle. This jet requires the least amount of adjusting over the rest. If your engine cuts out when you rev it up then you need to make this richer (higher number) if it bogs when you rev it up you need to make this leaner (lower number).
Needle jet: Controls 1/4 to 3/4 throttle. You can tell the most when this is out of tune when you do holeshots. If your engine is bogging when you dump the clutch on a holeshot or you are riding at a constant 1/2 throttle and you engine is bogging then you need to move the c-clip up one notch (drops the needle making it leaner). If the engine is cutting out in these situations you need to move the c-clip down one notch (raises the needle making it richer). Setting the c-clip in the middle slot is a good place to start.
Main jet: Controls 3/4 to full throttle and also affects all other jets slightly. This circuit is tested when you hold the throttle wide open. If your engine is bogging the you need to make the jet leaner (lower number) and if it is pinging or cutting out you need to make it richer (higher number).
Jetting can be very touchy and tedious. At any rate your engine should rev at a constant RPM at any throttle position and should rev up and idle down with no hesitations. Once you start getting your hands dirty and listening to your engine you will become an expert at jetting. Hope this helped and good luck!
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