I just spent the spring tearing down my engine, boring and honing (at a skilled shop), replacing all bearings, sealings and gaskets. And screws - those JIS ones are terrible, I put high quality screws everywhere.
Bike starts excellently and putters like brand new. Now I shall drive leisurely for five hours to break in the piston rings.
I do note however that when I put it in 6th gear, it hums. It sounds like straight-cut gears and vibrates, with an audible "mmmmm" sound. Not a trace of it in other gears.
What could be the cause? I did not leave any washers or whatever out, and as mentioned it runs perfectly smooth in all other gears.
One theory is that it is a harmonic resonance due to the gears having non-prime number of teeth.
If you have say 13 teeth on one sprocket and 20 on the other, every tooth on one will eventually meet every tooth of the other.
If you rather have say 10 on one sprocket and 20 on the other, the same tooth on one sprocket will ever only see two on the other, and over time they will wear to fit each other smoothly - but if you disassemble the gears and reassemble them so that they meet new different teeth, they will no longer match as well. I suppose they might wear together in time.
I never counted the cogs, but the handbook states that the gear ratio in 6th is "0.818". That fits with 9:11 and whole multiples thereof.
Which does have a prime number of cogs on one sprocket - so that should not be it. =/
What else can cause this?
