When I start my motorcycle in the cold (20 F) it makes quite a hard knocking noise at idle until it warms up. If I rev it up a bit, it seems to smooth right out, as it does when warmed up. Anyone know what this noise might be? (Specifics: it’s a 1980 Honda CB750, recently rebuilt)
not to sharp with bikes but it sounds like it could be oil pressure not getting to the crank right away. the oil moves slow in the cold.
Bump.
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I second the vote for oil pressure. I think that the valves are sticking a bit which is causing the knocking. Once the valves get lubricated and moving then the problem goes away. Just my WAG.
The only other thing I can think that it might be is something loose in the ignition system, like maybe a loose contact that becomes a good contact once the metal heats up and presses together. I would think that if this were the case just one rev wouldn’t do it, but that you would have to rev it up a few times so that the parts heated up. If one good rev will get it going, then it’s most likely something mechanical, and like I said if that’s the case my money is on the valves and oil pressure.
If this didn’t happen before the engine rebuild then I’d be suspicious about the oil pump. It might not have been put back together right or maybe it’s just on its way out.
Could also be piston slap.
Piston doesn`t fit perfectly until it heats up a little. Aluminum pistons will expand and the noise will go away.