My CB400F has a characteristic engine noise. With the engine warmed up it emits quite a loud, musical screeching - but only in a surprisingly narrow rev band around 3000 rpm. At 4000 rpm and above it runs smoothly and normally, with a reasonable amount of power given the age of the engine. The tick-over is also pretty smooth, with no obviously excessive chain noise evident. Also, no noise to speak of when the engine is cold. It’s just at 3000 rpm, after driving a bit, that you get the unpleasant screech. Not so much of a problem on the open road, but town stop-start driving is a pain.
I guessed a loose cam chain as the most likely culprit, so attempted to adjust the tension as described in the workshop manual, by loosening that lock bolt on the front of the crankcase. However, bad news: it turns out the thread is stripped. The lock bolt is permanently loose, and cannot be tightened.
Questions:
Am I right about the cam chain, as the source of the noise?
What prospects for fixing the stripped thread? Is re-tapping and fitting an over-sized bolt a practical solution?
What’s the chance of doing major damage by driving the bike in this condition?
Many thanks if you can help.
James
PS I am aware that the recommended cam chain adjustment method may not work in any case, on an old engine, but I guess I should cross that bridge when I come to it.
