In most cases there are three primary circuits:

1 – The Pilot Jet:  It is responsible for metering fuel or air from 0 to 1/8 Throttle opening. In most cases, this can be adjusted while the bike is running. (Preferably in conjunction with a gas analyzer.) The ideal CO level during this adjustment is between 2% and 3%. The hydrocarbons should be no more than 400 parts per million.

2 – The Needle:  The position of the needle meters fuel only, and contributes fuel from 1/8 to 3/4 throttle. By adding shims or changing needle clip position on High Performance carburetors or buy changing needle shape will affect all ranges of the fuel curve.

3 – Main Jet:  The main jet contributes fuel only from 3/4 to fuel throttle and will also will effect fuel contribution to the needle circuit.

Conclusion:  This is merely a snapshot of how a basic carburetor works. There are additional areas that will change the fuel curves such as, slide cutaway, accelerator pumps, external air bleeds, and venturi shape and diameter.

(It is highly recommended to use a gas analyzer when attempting to check the above.)