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Cb750 Clutch rattle

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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.

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