Carb Jetting

2 Stroke Engine tuning is about getting all the core components working together in symmetry. One of the main core components is the carburetor and jetting. Whether your machine is running a Mikuni, Keihin or Lectron carb, the basic principle is the same. You need to get the right amount of fuel and air entering the engine for the particular throttle position. Some 2 stroke engines can be very fussy about jetting especially when you consider the outside forces that effect the air fuel ratio, such as:

As a general rule, colder outdoor temperatures require bigger jets and smaller needles which allow more fuel to flow. Hotter weather requires smaller jets and bigger needles to lessen the fuel flow to accommodate for the air density changes.

As outdoor temps increase the air density decreases, so your mixture becomes "richer", meaning more fuel to air. As outdoor temps decrease the air density increases, so your air/fuel mixture becomes "leaner", meaning less fuel to air.

You're typical Mikuni and Keihin carburetors have four circuits to regulate air/fuel mixture at different throttle positions, (five if you count the choke as a circuit).

These, for lack of a better term "ranges" (0-1/4, 1/4-3/4, 3/4 - full) are not really sharp transitions but rather gradually bleed into each other via taper of the needle.