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Clutch Knocking

Rcb1000

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Just purchased a 1982 CB750SC, 26k miles. Making my punch list of items that need addressing. Bike idles rough but runs strong. Valve clear check 1st on my list. I did notice that at idle, there is a knocking sound coming from the engine that is reduced when I pull in the clutch or when engine speed us increased. Clutch does not slip, can spin the rear tire and functions best I can tell thru all gears. Any thoughts?
 
Idling rough the primary chain will knock at low rpm, it is hydraulically pressured. Carb sync will greatly help it or make it vanish if engine set to idle around 1000-1100 rpm. To hear the noise let out clutch in low gear without revving motor up enough to let it lug, you hear it then.

The clutch CAN rattle, there is a riveted shock absorber plate that the rivets get loose on and then they come off to go through the engine. Remove that double plate and replace it with two regular steels to get rid of it. The clutch action will improve too.
 
It's greatly needed on the 550 too, that one has no primary tensioner at all but sync will just about get rid of all of the rattle there.
 
Regarding the clutch knock, I had the same issue on my '82 custom. There are two sets of rubber isolators that you need to consider. The first set is in the main shaft, the one that connects the primary chain to the clutch. The second is in the clutch housing. I first tried rebuilding the clutch housing and replacing the rubber isolators in it. 4into1 has a rebuild kit for this. That update certainly helped but didn't completely solved the problem. I ended up cracking the case and replacing the isolators in the main shaft. In both cases, the rubber isolators that I removed were hard like marbles. After replacing both sets of isolators, the knocking sound went away, maybe not completely but certainly to a level that isn't too noticeable.
 
Yes, that rubber can make it knock too but they will always do it some at too low an rpm, my brand new ones did it with less than 2K miles on them. Because the oil pressure drops way back at under idle rpm, it can be literally nothing that low if you have oil pressure gauge like I did.
 
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