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.
