The "ole Skool Rule of Thumb" for centrifugal clutch ratios and tire diameter went something like this...
6" tire diameter = 2:1 sprocket ratio
So 18" tires, being 3 times larger, would require a 6:1 ratio, more like a 12T / 72T sprocket combo.
There is always fine tuning that can be done, but in the last century, tese are the numbers a lot of us used as starting points.
A "blue" crank is a definite indication of slipping, and imminent failure. Start with a NEW clutch and a 72T sprocket... you'll be surprised at the difference. You also need to allow for "hook-up," so the clutch can cool too. Constant stops, and hard accelerstion, will fry just about any combo.
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kliff
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