Step 3: DETERMINE THE CORRECT MAIN JET.
The main jet selection process is easy once you have the correct needle diameter or needle jet. You now only have to correct a rich condition from 3/4 throttle on up and you know what a rich condition sounds like. Your pilot circuit is correct and without bog.
- Replace main jet with one that is at least two sizes smaller.
- Needle clip in mid position.
- Start motor and run it on the stand.
By replacing the main jet with one that is too small, you are looking for a condition that is too lean. You adjust your main jet from a too small to lean condition.
Condition: Motor running and main jet in.
MAIN JET CORRECT:
Carburetor should run clean and crisp to full throttle.
Correction: None needed.
MAIN JET TOO RICH:
RPM reaches a peak slowly with a deep sound. Excess fuel and oil mixture at end of silencer. Spark plug fowls easily and is dark in color.
Correction: Mikuni replace main jet with next leaner and test again. Keihin replace main jet with next leaner and test again.
MAIN JET TOO LEAN:
RPM reaches a peak quickly but erratic. A quick full snap open of throttle causes the motor to hesitate BEWAH sound or a complete bog. Motor sounds like it has a ring to it. End of silencer white. Spark plug is white in color.
Correction: Mikuni replace main jet with next richer until the BEWAH bog just barely goes away, then replace the main jet with the next richer and run it. Keihin replace main jet with next richer until the BEWAH bog just barely goes away, then replace the main jet with the next richer and run it. The emphasis here is find a main jet that is just rich enough to allow you snap the throttle wide open without the motor bogging as a result of the main being too lean. Should be a quick crisp throttle with no hesitation.
Step 4: DETERMINE THE CORRECT NEEDLE TAPER AND CUT AWAY.
This step in the jetting process can be made very simple if you remain close to stock. However, your needle taper is adjusted for 1/2 throttle to 3/4 throttle. Start off with a rich taper (shallow taper angle) and keep going leaner (steeper taper angle) until it will not maintain constant RPM at 1/2 throttle (runs erratic). Go back to the leanest taper angle that ran the smoothest at 1/2 to 3/4 throttle and that should be the correct taper.
The needle taper final test should be under track conditions with the greatest effect entering and exiting corners. Do not change the needle diameter or needle jet size during this process because that has already been determined. Adjust taper and throttle cut away only.
Throttle cut away effects from idle to 1/4 throttle. The correct cut away will maintain steady 1/8 throttle with quick throttle response. Generally the stock cut away is very close. Experiment with different cut away until it maintains the best response to 1/4 throttle.
That's it, if you spend the time jetting correctly, the benefits you will gain definitely out-weigh way the time spent. Take the guesswork out jetting by following a procedure that has been given or one you have laid out yourself. Keep the black magic process out of your tuning tricks and you will be better off for it.
From an article by Don K. Courtney. Link is dead now.