Bag of bolts

[Gacataki]

Hi.

Please help. I have a 1980 CB750 Custom that runs beautifully and smoothly.

But when it idles below 900 rpm, the motor sounds like a bag of bolts. If I set the idle to 1300 rpm the sounds go away. I was told by an 'old school' mechanic that this is pretty normal.

Any thoughts please.

 

[SenseAmidMadness]

My bike does exactly the same thing.

It's the primary chain tensioner. That tensioner is driven hydraulically off of oil pressure. Low idle results in low oil pressure and the tensioner can't function properly with low oil pressure, so the primary chain (the chain that transmits torque between the engine crankshaft and the transmission mainshaft) rattles around.

The factory service manual has the idle a spec bit low in my opinion -- you should aim for 1100, the high end of specification, when the engine is completely warmed up. I find that my bike runs a lot smoother and happier there.

 

[brettp]

Ditto on the above. I think mine idles at 1200. I've done the can chain tensioner adjustment process a few times and it still makes that chain whipping noise.

 

[Gacataki]

Thanks guys