After the exhaust pipes (motorcycle) are cool to the touch, soak the crud with a petroleum based product over night or longer. (Use an old cotton sock or similar - non-dyed - rag). I have tried: SAE 20 weight oil, WD-40, carburator cleaner, gasoline, and fuel oil. A few other solvents and cleaners have limited success: acetone, paint thinner, mineral spirits, wood alcohol, chrome cleaner, and rubbing compound.
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albertjames
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