® CBX "clutch rattle"

CBX clutch rattle. Everyone in the CBX community has heard of it, all the forums seem to be talking about it, and folks here and there are modifying their clutches to try and get rid of it. But is there actually a little more to this?

Primary chain
First, it's rarely the clutch. A deep seated lightweight knocking at idle that disappears as soon as even slight throttle is applied to lift the engine rpm off of idle, is what we're talking about here. Rattle is a good word for it. This is not rod knock or cam chain noises, piston slap, piston pin knock, valve tick or what-have-you. But, it's not the clutch either. The fact is, it's actually the primary chain that's rattling.

1970s Hondas are famous for this noise, mainly because their inline engines have chains between crankshaft and transmission, not gears as they would in later models. And the smaller ones don't even have tensioners. The larger SOHC 650, 750 and DOHC fours and CBX, though blessed with a tensioner, can still make the noise. In the case of the SOHC 750, its dry sump design further accentuates engine sounds. Thus the phrase "primary chain rattle" is virtually synonomous with the name Honda.

What makes the chain rattle?
Every veteran Honda mechanic knows what causes primary chain rattle--less than perfect engine tune. Primary chain rattle results when the engine's cylinders are "fighting" each other, causing engine torque to flow unevenly, thus the primary chain to whip and twist more than usual. The key to minimizing primary chain rattle therefore is to make sure each cylinder is working the same. Whether low and/or uneven cylinder compression, tight valve clearances, imprecise ignition timing, dirty carburetors, or simply carburetors that could be better synchronized, primary chain rattle goes away when these things are set in order and the engine is running as smoothly as it was designed to.

The clutch
So the $64,000 question: Where is the clutch in all this? The clutch's role is indirect at most. Because many vintage Hondas can, given enough miles, wear out their clutch basket rivet holes, leading to a loosened clutch-to-transmission drive gear at the back of the clutch basket. The gear doesn't get loosey-goosey, but it can be made to move perceptibly if you remove thd basket from the engine and hold the basket in one hand and try to rotate the gear in the other. I remember doing this check on many of the tiddler fours. But despite detecting some slop, mechanics seldom did anything about it. It was just no big deal. They all did it. And they were all quiet after a professional service. This looseness, if severe, can add to the primary chain noise, but it's rare. And, whether it contributes to noise or not, in these relatively low-powered engines a loose clutch gear is not going to self-destruct. It is a potential sound issue only, assuming you are not doing dragrace launches, wheelies and burnouts.

Summary
So the infamous CBX "clutch rattle" is actually primary chain noise. It is caused by very worn primary chains or a whipping and jerking chain due to imprecise engine tune. In some extreme cases it can be accentuated by a worn clutch basket, and though annoying, this is not destructive. And now you know.


Last updated November 2022
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