Step one, check for air leaks, make a leak down tester, if it can't hold 4 psi for 20 min you are trying to piss up a rope. This is LITERALLY the one step on a 2 stroke you can't neglect. It's that important.
Then follow this guide.
www.shiny-red.net/guides/keihin-carburetor-jetting/Step 1: DETERMINE THE CORRECT NEEDLE AND OR NEEDLE JET.
This is the most important step in jetting your carburetor!
Remove the main jet.
Place needle clip in mid-position.
Start engine and run it on the stand.
Condition: engine running and main jet out. Needle or needle jet is correct: Carburetor should run clean to approximately ¾ throttle. From ¾ throttle to full throttle, the engine should start to break up as a result of too rich condition.
Correction: None needed.
Condition: Needle or needle jet is too rich. Carburetor runs clean to approximately ½ throttle but breaks up before ¾ throttle as a result of too rich condition.
Correction: replace needle with next leaner diameter and test again.
Condition: Needle or needle jet is too lean: Carburetor runs clean beyond ¾ throttle and has an erratic throttle response.
Correction: replace needle with next richer diameter and test again.
The emphasis here is to find the correct needle or needle jet diameter, which will allow more fuel to pass than is needed but not so much that the needle itself has no control below ¾ throttle.
Step 2: DETERMINE THE CORRECT PILOT JET.
Make sure the scooter is warmed up if at all possible.
Main jet out.
Needle clip in mid position.
Turn air screw all the way in then ¼ turn out.
Start scooter and run it on the stand.
Adjust idle so the scooter will just barely idle.
Condition: scooter running and main jet out.
PILOT JET CORRECT: With one hand on the throttle maintaining RPM at approximately ⅛ throttle, turn air screw ¼ turn at a time clock wise until you bottom it out. engine should become slightly erratic and you should have to play with throttle to maintain RPM. Start turning air screw counter clock wise, ¼ turn at a time until you have reached 2 ¾ turns out. Between 1 ¼ and 2 ¼ turns, your engine should have reached its highest RPM maintaining a steady throttle. Adjust air screw again between 1 ¼ and 2 ¼ until you have determined highest RPM. Quick throttle response should be clean without bog.
PILOT JET TOO RICH:
RPM does not reach a peak between 1 ¼ and 2 ¼ turns, stays the same or keeps rising out to 2 ¾ turns.
Correction: replace pilot jet with next leaner and test again.
PILOT JET TOO LEAN:
RPM does not become erratic and engine maintains throttle when air screw is turned all the way clockwise.
Correction: Keihin replace pilot jet with next richer and test again. Remember, with a steady throttle approximately ⅛, there should be a distinct difference in RPM from 1 ¼ turns to 2 ¼ turns if the pilot jet is correct. The emphasis here is to find a pilot jet that will run crisp without bog and without the main jet.
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 ¾ 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 scooter 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: scooter 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: replace main jet with next leaner and test again.
MAIN JET TOO LEAN:
RPM reaches a peak quickly but erratically. A quick full snap open of throttle causes the engine to hesitate BEWAH sound or a complete bogs. Engine sounds like it has a ring to it. End of silencer white. Spark plug is white in color.
Correction: 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 engine 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 ½ throttle to ¾ 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 ½ throttle (runs erratic). Go back to the leanest taper angle that ran the smoothest at ½ to ¾ 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 ¼ throttle. The correct cut away will maintain steady ⅛ 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 ¼ throttle.
That’s it, if you spend the time jetting correctly the benefits you will gain definitely out-weigh way the time spent.
