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Cb750 Clutch rattle

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Martin6:
I agree regarding general noisy/ lumpy tickover (if that is your issue, rather than a clutch problem) and add that this is true if, to varying degrees:
-Cylinder compressions are matched
-No differences in cam wear across the 4x2 set of valves
-Valve adjustments are spot on
-Cam and primary chains are adjusted / in spec
-Ignition is equal across all 4 (plugs, leads, caps)
-Points and timing are spot on
-Advance mechanism isn't worn
-Is the tacho actually accurate
Probably things I've missed.

Too many variables for my old engine to idle at low indicated RPMs. I set it up to pull nicely through the gears at varying throttles, with good plug colours, then set the idle where it is comfortable and smooth, which was about  1200 -1250rpm.

SeanFD:
I completely agree with the last sentiment.

I also set the idle where it is consistent and smooth - about 1200 rpm.

K2-K6:

--- Quote from: Martin6 on November 17, 2025, 11:37:24 PM ---I agree regarding general noisy/ lumpy tickover (if that is your issue, rather than a clutch problem) and add that this is true if, to varying degrees:
-Cylinder compressions are matched
-No differences in cam wear across the 4x2 set of valves
-Valve adjustments are spot on
-Cam and primary chains are adjusted / in spec
-Ignition is equal across all 4 (plugs, leads, caps)
-Points and timing are spot on
-Advance mechanism isn't worn
-Is the tacho actually accurate
Probably things I've missed.

Too many variables for my old engine to idle at low indicated RPMs. I set it up to pull nicely through the gears at varying throttles, with good plug colours, then set the idle where it is comfortable and smooth, which was about  1200 -1250rpm.

--- End quote ---

I don't doubt there's variance in some of those areas, but people routinely hold that a csrb sync is the answer in whole .... and also adjusted with all those "faults" potentially contributing.

There's no adjustment or mitigation for primary drive chain on these, just fitted and hopefully of good serviceable condition, that's it.

Any technical effects can really only be changed by understanding the true original problem though. The clutch doesn't rattle when closed, its the drive train around it, both before and after, that input coming from crankshaft fluctuating frequency from uneven burn cylinder to cylinder  .... importantly, at idle rpm.

Carb sync is given as overall control of this, when it plainly isn't. The performance at these low rpm is substantially down to the fuel metering. That's after other parameters are set correctly. Its in the first Honda workshop manual and subsequent iteration too. How to set it is well layed out, often ignored. It does ask for use of accurate tachometer and not the one fitted to bike though, this should be available in small digital volt meter if really someone wanted to follow the instructions verbatim.

The idle jets, air passage and their setup are there to provide this compensation in virtually all carburettor of this type, there being too many variables in production manufacturing to support a fixed air fuel ratio design with nil facilities to optimization process. Most choose to ignore the reality of the design.

Mitigation, in raised rpm etc, is just that. It doesn't answer the question of root cause.

Adding component to the clutch pack won't change this reality (with regard to original question at thread start) as they are aimed at the clutch action as it takes loading when driving away from static starting.

It may be difficult to set jetting in absolute terms, given that it's time consuming, and the apparent interaction with modern fuel, especially when used sporadically. It is nevertheless the primary cause of this characteristic though.

K2-K6:
I've put it on here before, thar the idle circuits are not absolute in consideration of setting their air screws.

The fixed point being the bore size noted on the jet, as they will, all four of them, flow to this number at their maximum. The maximum comes when the air through the screw controlled supply (for most) reaches saturation and the suction on the jet in the float bowl goes full "bleed" which lays approximately at 1/3 ish throttle opening. The aircrew position can delay or advance that happening, but it won't change the peak flow volume. To do that, tbe jet bore would need to be swapped.

The bottom on idle circuit flow though, is much more affected by air screw position. Its a very fine proportional metering method to control fuel delivery at tiny demand.  Which is how it affects cylinder speed at those low rpm .... but importantly it changes almost nothing it absolute contribution to mixture as you move away from tbat throttles closed position.

Its not a trade off against correct jetting for engine power and response, just a trim point at that low rpm, which disappears as more throttle is used. The two live together happily in one transitional design, they are not exclusive. 

Individual air screw adjustment IS the acceptance of other variables such as listed in previous postings. It is when new and "perfect" it still is when old. If it doesn't respond to acceptable practice layed out in manual, then there's underlying issues  .... which are the cause of "clutch rattle" the driveline chattering when the engine is idling unattended.
Lifting the base rpm just alters the the amount of time the crankshaft can react to input, taking its oscillation frequency out of range of the following driveline components such that  they don't respond in time to rattle.  Which is fine if that's what you want, but not fixing anything as such.

royhall:
It's not exactly the best system to have two simplex primary chains tensioned by one tensioner. With the best will in the world these two chains are never going to be the exact same length, nor will they wear the same. So that one tensioner is going to cause chain slap that will resonate through the drive train. Although in practice the chains last very well, it's hardly a good design. So really anything that can be done to keep the rattling at bay is no more than a workaround to limit the effects of that bad design.

People may know otherwise, but I personally have never seen a 750 that doesn't rattle to some degree, all you can do is calm it down with careful setup. In my experience the electronic tacho setup of the jets is very difficult and fiddly to do and barely has any effect on the noise whatsoever. Balancing the air flow across the 4 carbs does appear to smooth out the rattle. As I say, other people may have different experiences, but that is what I found. I would be happy for someone to correct me on this with a proven procedure that works as that rattle is a pain.

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