Cb750 Clutch rattle

[royhall]

Morning. Had a quick site search and didn't come up with an answer. Did anyone ever come up with a truly workable solution to the famous CB750K clutch rattle. Somebody mentioned a Barnett kit, is that a real thing and does it work. Cheers.

[Martin6]

I went with 20W50, balanced carbs and an idle c.1200rpm, with standard clutch.

[Skoti]

Yep,
Just balance the carbs as Martin suggests, and the rattle will disappear.
I use Morgan Carb Tune for this, cheap and easy.

Good luck

[SeanFD]

I came across this a while back, and again a few weeks ago:

[K2-K6]

There's not a cure from the clutch as I experience and understand it.

The rattle is from clutch, gearset and primary drive chain in oscillation that you can here.

The clutch, when left alone (no lever pulled) is effectively solid between the plates, this to be effective in transmitting drive. They don't rattle.

The oscillation comes from crankshaft frequency in shifting, that a response to imperfect cylinder to cylinder combustion balance.

This effect is amplified on 750 as its primary drive chain has no shift in link to sprocket tooth dimension, its stays exactly the same relationship  - unlike the 500/400 have "silent" type that compensate tooth form according to rpm and centrifugal effects.

Further, the 750 has a chain "tensioner" wheel on a sprung stick arrangement for primary, but that's in no way damped.

The crankshaft oscillation will "shunt" back and forth through that chain, and its slack/backlash to excite the gearbox components, this acting as something of a remote flywheel from their all up mass in rotation.

Short version, uneven burn at tickover (from one cylinder to another) will rattle the gearbox and components leading to that. Its NOT the clutch 

Carburettor sync is only part of the answer as it just syncs the flow through each carb, for air flow.

The icing on the cake is fine metering through each idle jet individually to give, as near as possible, even piston speed on power stroke from burning the precise amount of fuel to do that. Its in the Honda manuals how to set it that way.

Incidentally, this is exactly what modern fuel injected engines do for that smooth idle, only monitored through crankshaft position and speed on the fly, that to measure each individual cylinder pulse as the sensor picks up error. The injection pulse is then trimmed to close that loop, if a cylinder pulse arrives too early it trims the fuel to delay next event. Too late and it will add fuel to give more energy and speed it up. Tuning idle air jets does this in aggregated total.
If current FI runs out of adjustment, but still the pulse is out of range, it'll generate a CEL check engine light warning.
The carbs set not perfectly, just shake the crank, and rattle the components.

[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]

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.

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.

Sent from my Pixel 7 Pro using Tapatalk

[K2-K6]

Definitely see your point of view Roy.

For posterity on forum, its worthwhile noting that there's not a mechanical cure for it. The clutch pack "adaptions" primarily focused on torque take up as the clutch is released. When it's closed, effectively one rotating unit that has no means of rattling.

The cush drives, in any clutch, are for frequency buffering to stop crank pulses being a problem as they go down the transmission line. Loose though will contribute to this effect in system rattling, although not specifically designed to cure or mitigate while sitting there at idle.

Yes, the original design of chain (obviously very early in mass produced 4 cylinder design ) is quite crude and with rudimentary "tensioner" that has no damping action at all. The origin though still comes back to crankshaft giving lumpy delivery when sitting idling.

Years ago, talking to a builder of these for sidecar competition, he explained that they had really short chain life in typical use as the change down engine braking (to alter the outfit attitude in steering) really caned the chain, particularly as it effectively pushes the wheel out from tension on that "slack" side of chain run, to then leave the top (normally pulled tight) run flapping like a flag.  His experience was, if left too long the chain would override the sprocket teeth and snap under hard use.
Also why cam chain systems have a curved tension side, and adjusted guide opposite on return run of chain.

Of course they went to hyvo for 350/400/500 as they effectively run with very low slack that could amplify oscillation potential.

Honda actually modified the primary drive in last iteration of this bottom end too. The RCB endurance racer was this bottom end with bespoke twin cam 16v head, in which they converted to gear primary, with a idler intermediary gear to correct the drive direction when going from chain to gear pair. You can see they came across limitations here, that led to further development in producing next generation of road going twin cam tbat came next. Acknowledging your observation here effectively.

[Laverda Dave]

What a great explanation Nigel especially with the racing history to give it a bit more context.
Thats why I love my VFR, no chains flapping around (apart from the rear drive chain!).

[K2-K6]

Yes, definitely a very high water mark in Honda history those gear-cam V4  sounding absolutely fabulous too.

Good you got to keep your one Dave what with the U lez scare etc.

Speaking to a local accredited tester for that scheme, they tell me that virtually all bikes tested from before emissions classification will generally pass. The critical limit was never a problem for that era as they don't ordinarily run lean/hot with other exhaust gas treatment not fitted. They were, of course not classified as routine although that would have qualified as below limits anyway.

[Laverda Dave]

Mine failed on the first attempt Nigel (and it is PFI not carbs so I expected it to pass). It failed on a closed throttle after an acceleration run, the injectors squirt fuel into the engine on the overrun as they are designed to do, the fuel didn't fully burn and it was 0.1 over the limit! The tester (an ex racer in Stevenage where I got it tested) used his laptop to remap the injector sand put a bit of redex in the tank in order to solve it and I got a pass following another run on the dyno.
The tester said the TfL dyno programme they have to run the bike to requires the bike to accelerate from very slow speed up to 40mph for a few minutes and then back down to virtually walking pace. It’s all utter pants, show me anywhere in London where you can travel at 40mph for a few minutes especially considering everywhere is LTN's, camera's and traffic jams!
It is all a big money making scam by our wonderful Mayor.

[K2-K6]

I recall the overall experience of testing, and ultimately successful outcome Dave.  Thought it was very good you've been able to keep such a nice bike that's been cared for so well.

Thats a cute adaption used though, tbe redox dosing    it may have cleaning effects, but just as importantly it'll lower combustion temperature (a key driver of the output) by being non available to burn in petrol combustion, illustrated by smoke usually.  It should keep combustion temp down a little by not contributing to burn load for each stroke its used. This to help meet the demand of certain measured parameters.