Naturally aspirated 2-stroke engines use a carburetor to deliver a mixture of air and fuel into the engine. The process of jetting, or tuning, a carburetor adjusts the ratio of air being mixed with fuel. Getting the mixture right can help improve fuel economy and give an increase in speed. It can also make your engine run more smoothly.
Before tuning your carburetor be sure the rest of your engine's components are in full working order. Among other things, check that the spark plugs are working correctly, the air filter is clean and the head gasket is not blown. There is not much point in going to all the trouble of jetting your carburetor if your engine is not in good running order.
What might be a great tuning setup for you may be useless for someone living in a different part of the country. Altitude and humidity are an important factor as they directly affect the amount of air available for the engine. The higher above sea level you live the less air will enter the carburetor, and the jets should be adjusted accordingly. Similarly an engine tuned to run perfectly in summer may run awful during colder, winter months. Once you have the carburetor tuned to perfection remember that at some point in the future you will have to adjust it according to the local conditions.
The carburetor contains a pilot jet, a needle jet and a main jet with each jet working at different engine speeds, although they do overlap each others range. The pilot jet is in use when the engine is idling up until the throttle is around 1/4 open. The needle jet's range is from 1/4 throttle up to 3/4 of throttle. Above 3/4 throttle and the main jet comes into use. Check the stock specification of each jet and buy three jets that are larger and three jets that are smaller than each type. You now have six new sizes for each of the jets and can adjust the carburetor over this range.
The ideal ratio of the air to fuel mixture is 12.5 to 1, this means that there is 12.5 times more air for every one part of fuel. Increasing the size of the jets requires that you also increase the amount of fuel entering the carburetor. Keeping the mixture as close as possible to this ratio will enable you to get the best tuning setup for your carburetor.
