Try the next size smaller pilot jets.

Remember, carb tuning relates the jets to the throttle position, not the engine RPM. The pilot jets are for the lower throttle position, needle adjusts the mid throttle setting, and the mains are for wide-open throttle.

9 times out of 10, the plug fouling problems are due to the lower throttle position settings being too rich. Especially on the MC28. I am not that far above sea level, in a very dry area, and my bike runs too rich on the 38 pilot jets. I cant imagine the problems others are having at higher altitude and/or more humidity.
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Paul Herr
'88 FZR4/GSXR/YZF Frankenbike
MY BIKE PICS