Three adjustments in fuel metering, pilot jet and screw, needle, and main jet. All three have an effect at all engine speeds/throttle settings. Adjustments to the pilot jet and screw have the most effect at low throttle settings. Needle adjustments have the most effect at mid throttle settings. Main jet adjustments have the most affect at high throttle settings. however, the main jet does affect part-throttle operation, but not much. The pilot jet and screw do affect operation at high throttle, but not much.
Start with the main first. adjust for proper mixture at full throttle. Then work with the needle height for best part-throttle engine response. finally, work on the pilot and screw.
Then go back a tune the high speed again. follow the pattern through several cycles, making finer and finer adjustments until maximum performance at all speeds and throttle settings is obtained.
Below is an example of the above process applied:
Thanks, guys. It just seems so many people think rejetting a carb is voodoo or something. It’s not. It just takes patience and persistance. I like to run the main jet a tad richer than optimum performance to lower engine temperature. I do a lot of highway at sustained high throttle settings. Many miles of the East Texas and Big Bend rides consisted of mile after mile of deep, loose mud and/or sand. Maintaining 40-45mph required 4th gear and near full throttle. A lean main would not have caused a problem in East Texas with the temperatures in the ’40s, but Big Bend had temps in the high ’90s out in the desert.
I usually choose the main by reading plugs after some full-throttle plug chops. I had to open the carb twice to jet the mains.
Then I lean out the midrange until I feel lean surge at part-throttle cruise (35-40mph on a TW). Lean surge feels like a miss to many people. I keep adding washers under the needle until the lean surge goes away. I had to open the carb 4 times to get this right.
Finally, I richen the idle mixture screw until the bike cold starts easily and accepts throttle quickly from idle when warmed up. If the bike is too lean at idle it will not cold start easily and will die if the throttle is opened quickly. I had to open the carb 6 times to get the idle mixture right.
All together, I had to open the carb 12 times to get it right, and this was less than normal.
If the bike is too rich at idle or midrange, the engine will hesitate, then accelerate, when the throttle is snapped open.
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WA Riders 