With the introduction of the new Yamaha motorcycle 2008 model line, it looks that the end of the good old carburetor is in sight. The carburetor as we know it today was invented by a Hungarian engineer back in 1892.
But enough about the sentimental part, and back to our task: tuning the main jet circuit of a regular carburetor.
Tuning or adjusting the main jet circuit is where the most confusion seems to be. The main jet really doesn't come into full play until the last quarter turn of the throttle. When the needle rises in the tube far enough that gap is equal to or greater than the size of the main jet, the main jet becomes the metering device for fuel flow.
This is where the meat of the power lies. At full throttle, the amount of fuel pumping into the engine can be startling. If you've ever seen a bike equipped with a velocity stack on the dyno, the amount of fuel getting sucked into the motor is pretty impressive.
The number stamped onto the jet is the jet's size. This size can be in millimeters, thousandths of an inch, flowing rate, or a number of other interesting units of measure, it depends on carburetor manufacturers. Generally speaking, the larger the number, the bigger the jet, and richer it will be.
Tuning the main carburetor jet circuit is easy enough. You have a rich and lean condition. The best result is to set the main jet intake to the setting in which you can get the highest rpm at full throttle while still getting a smooth running engine at idle. Also if with a warm engine and you open the throttle from zero all the way open and you feel the engine is somehow holding back... you have set the main jet circuit to rich, it is possible that you get the highest rpm at this setting but you losing out on power at lower rpm, in racing situations you opponents are already gone before you get at full rpm.