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If I ever do.....No smoke and no leaks--have you run a compression check on it?
After being serviced, does it run correctly?
When changing the air filters, did you ever notice the "clean side" of the airbox being dusty?
Have you checked all clamps and connections between the air filter and the motor?
Ever see any metal or anything unusual in the oil?
Anything unusual regarding the fuel you use?
It is a real mystery given 1) how meticulously you have maintained it, and 2) it is a Honda. Please report back when you find the cause.