Topic: '81 CB650C won't idle and knocks like crazy

[unwind101]

My '81 CB650C ran great until sometime last year when it rather suddenly started to idle rough and knock like crazy at low RPM.  I parked it, drained the carbs, and let it go for the winter.  Now I have drained the tank, put fresh gas in, started her up, and of course the problem didn't 'magically' go away.  So if I give it plenty of gas it seems OK at 3000rpm and above, but when I let off the throttle it immediately wants to stall.  When I give it gas again it knocks like crazy until I get it up to about 3000RPM again, then it runs fine.  Sounded like the timing was too advanced, but I checked the timing with a timing light and that seems fine (when I first started it cold w/ plenty of choke it sort of idled and the timing was correct).  Spark advance seems to be perfectly fine as well.  Cam chain hasn't slipped.  So what would cause it to run like c&*$! and knock so badly below 3000rpm?  Thanks.

[phil71]

if you have dirty pilot jets , some cylinders may not run at idle. When one or more cylinders doesn't run at idle it throws off the balance, and shakes the primary chain, and then the clutch will amplify that sound. This is one possibility, but seems likely. A more serious and permanent problem would likely not dissipate at speed. Except maybe a cracked piston ring. Start with the easy, and cross your fingers.

[cb650]

The 650 clutches are noisy when everything is good.