SOHC.co.uk Forums > CB350/400

400 four knocking noise.

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K2-K6:
I'll have a go at describing the sequencing there with what you've got.

The drive from the crank passes to the clutch, this is essentially another flywheel albeit remote from the crank. A flywheel wants to spin at a consistent rate (the main reason for putting one on a crank) and imparts it's inertia backwards down the primary drive route if there is a change in crank speed.
Bare with me while I try not to confuse myself here!
When the engine is running then for one crank revolution you will get first one cyl firing while it's paired piston is at tdc at no compression, and the other two pistons at the bottom of the stroke....this is then reversed for the other two cyl.....now these small four cylinder engines sound very smooth but at low rpm there is significant crank speed fluctuation when ever one pot fires as it first comes up to compression (only one cylinder doing this at a time remember) the crank slows, followed by acceleration as the ignition fires that cyl.
That speed change comes through the primary drive BUT is opposed by the clutch which is happily spinning consistently as it has no balance issues.

If you think of what happens to the primary chain top run as the crank slows but the clutch keeps spinning it should go slack momentarily....followed by a whack as the crank speeds up faster than the clutch inertia.......it's this, I believe, that the clutch cush drive is supposed to mitigate, i.e. when the chain twangs it's the cush that should allow some give to stop that being a mechanical clunk that you can hear from outside.

So if the cush drive has gone hard and it should have some elasticity when new, then the small shock load it should be absorbing is just not happening.

You can even get a good demonstration of exactly this process if you put most bikes on the centre stand and let the engine idle.....put it in third or fourth gear and watch the final drive chain as it clatters up and down (the rear wheel being a sort of flywheel will replicate the primary drive inertial effects only on a larger scale).

So in principle I'm saying that there is not enough controlled movent in the clutch cush drive and it's most likely that most older examples would be similar.

See what others reckon.

Nigel.

Lobo:
Hi Nigel / OddJob,

First up that thrust washer thought is giving me a sleepless night; I didn't take one off, or put one back. Bollocks! In truth I'm thinking it simply remained in place on its shaft whilst I verified the no-basket rattle fix - mainly because I DO remember, on reinstalling the castellated nut only a thread or two (at most) exposed post tightening. Working in a bit of a rush etc!

Nigel... thanks for the explanation - which all makes sense....apart from the 'bag of nails' cacophony - ie a mixture of metallic sounds. Is it simply the components of the basket make-up, all the rings etc rattling individually - there are for sure an 'awful lot' of parts.  This all said, I would have thought an engaged clutch (ie lever in normal position) would be tight enough not to allow individual components to rattle.
Have taken a punt on a 2nd hand basket, and by way of a project plan to order a shiny set of those red cushes above.
Cheers for your thoughts / help,
Simon

Clem2112:
I follow that...

The generator rotor also adds mass to the crankshaft mass to help reduce the speed fluctuations but there still isn't much flywheel effect overall.

The noise may also be worse if the cylinders are not all producing the same shove at idle?

When I had my original bike the knocking noise was less evident when the carbs were balanced and the timing was spot on.
Some bikes are also a lot quieter with the clutch disengaged. This de-couples the gearbox which mean less mass to be sped up and slowed down with each rev of the crank to reduce the noise.

ttr400:
Hi Lobo, sent you a PM. seemed to hang up when sending so not sure if it went through?

Kevin

ttr400:
Hi Simon, thanks.  sent you a PM.

Damper rubber kit, posting today via registered airmail.

Kevin

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