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 what is it, and what should you do about it, if anything?
First, let's clear up something important. 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. And, the fact is, it's actually the primary chain that's rattling, not the clutch.
At one time Hondas were famous for this noise, mainly because their smaller inline fours had primary chains but no tensioners, and typically ran around half-maintained. The larger SOHC 750, though blessed with a tensioner, still made the noise, because its tensioner was weak, this bike too was poorly maintained, and undoubtedly also because the 750's dry sump design accentuates engine sounds. The "phrase primary chain rattle" was virtually synonomous with the name Honda because it was unusual at a time when other Japanese bikes had quieter gear primary drive. It continues to stand out today.
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.
So the $64,000 question: Where is the clutch in all this? The clutch's role is indirect at most. Many vintage Hondas can, with enough miles, wear out their clutch basket rivet holes, leading to a loosened clutch drive gear at the back of the basket. The gear doesn't get loosey-goosey, but it can be made to move perceptibly if you 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. Such looseness can presumably add to the primary chain noise, but in these high mileage Honda fours I never observed that it actually did. Moreover, in these relatively low-powered engines a loose clutch gear is not going to self-destruct. It is a sound issue only, assuming you are not doing dragrace launches, wheelies and burnouts.
So the infamous CBX "clutch rattle" is actually primary chain noise. In some extreme cases it can be accentuated by a worn clutch basket. It is not destructive. And, it is a result of engine tune shortcomings. To emphasize this main, final point, consider that a properly set up CBX engine and carbs will idle so low, so smoothly and so quietly the bike's tach can be made to read all the way to the bottom -- yes, zero rpm -- while the engine continues to idle without hiccuping or exhibiting any undue noise. Not that you would want to maintain this low an idle, but just to show how smooth this engine is when correctly set up.