There are 4 main circuits used to optimize gasoline delivery and therefore engine performance. You might adjust the air screw, adjust the jet needle valve clip position or exchanging the pilot (slow) jet, main jet, throttle valve (slide) or jet needle for one of an appropriate size to achieve the correct stoic metric ratio. The stoic metric ratio for 4 stroke engines is 14.7:1 (14.7 parts air, to 1 part fuel) although some people believe going rich or lean at certain points can aid performance. Ie. going as rich as 12:1 can be beneficial to acceleration, and going down to 15/16:1 can aid fuel economy at idle revs. This is all dependent on the engine, even 2 identical engines could require different settings to achieve there relative best performance

The different jets in a carburetor are targeted at specific throttle openings. But none act idependently, this helps to achieve a smooth power delivery, but plays havoc on the tuner in some instances. See below for a list what the jets are targeted at.

The air screw is most effective between idle through 1/8 throttle. The pilot (slow) jet is most effective between 1/8 through 1/4 throttle. The slide valve is most effective between 1/8 through 1/2 throttle The jet needle is most effective between 1/4 through 3/4 throttle. The main jet is most effective between 3/4 through wide-open throttle. Pilot Jet/Air Screw

The pilot jet, or slow circuit, can be adjusted by either swapping the pilot jet for a smaller or larger jet, or by adjusting the air screw, the air screw controls the flow of air into the circuit. Usually turning the air screw in (clockwise) will richen the mixture (reduce the amount of air) and vice versa.

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