Topic: 1977 CB750F Engine Idle Noise

[superbike]

I just picked up a 1977 CB750F with just under 9000 reported miles.  Getting to the point, at idle there is transmission noise, increase the rpms and it disappears and the motor sound fine. Under power while driving there is a slight knock that can be felt in the foot rest.  Otherwise the motor shifts and runs great.   The noise sounds a bit like a cement mixer and is not completely unlike that of a 78 CB750F that I run, but more pronounced.  Insights would be appreciated.

[superbike]

Could the noise be something with the clutch?

[riffman12]

hmm I assume you've checked the easy things? Adjusted the valves? Adjusted the cam chain tensioner? Drained the oil and checked for chunks?

Next I would probably pull the pan and check for any chunks in the screen.

[mj1176]

It can be hard to nail down on these, but the clunky sound during idle that goes away with increased RPMs is classic primary chain slap description. Mine does it, all the 70's CBs I've seen do it to some degree, it's just a symptom of having a 35-40 year old primary chain. It can be hard to diagnose if everything else on the bike isn't in good shape; do a valve / cam chain adjustment and carb sync and see if it gets better. You might also be hearing a little clutch basket rattle, which can make it harder to tell what you're hearing.