Gearbox knocking

[Stevie63]

I have just taken on a '79 CB750 DOHC and eventhough the motor is running there is a knocking noise coming from the gearbox. Having drained the sump down, run Engineflush through,cleaned the hyvochain hydraulic adjuster there is still the knocking at idle revs. Has anyone else come across this problem?

[LukeM]

Hmm...gearbox, or perhaps something around the crankshaft? It's possible there's a bearing going bad (rod or main bearing on the crankshaft).  Sometimes they'll knock or rattle at idle, and be less noticeable when you're on the gas.

Might want to check your oil pressure as well.  Low oil pressure will lower the spacing between components, including bearings.  

Hope this helps.  I'm hoping you won't have to yank the engine and rebuild it.  
Luke M

[Rodgil]

Its not uncommon for the noise to be caused by backlash between the motor and gearbox, at slow idle. This can be made worse by an uneven idle, by the carbs being a little out of sync. If there is no noise at anything above slow idle, it is probably not worth worrying about.

[TOOLS1]

I have to agree with Rodgil. Carbs out of sync can make an engine sound like it's about to fly apart.
TOOLS

[Re-run]

Yeah thats for sure. Loose cam chain and out of sync carbs can make your bike sound like a washer at idle.

[theban]

http://www.youtube.com/watch?v=YVPpScwzbfM

could this sound in my 1981 cb750F come from backlash due to out of synch carbs?