SOHC.co.uk Forums > CB350/400

400 F2 Noise

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cliff7:
Thanks for input. Carbs were bench synced about 50 miles ago and very slightly tweaked by me using vacuum gauges. As an (ex) car mechanic I am not too bad at wielding spanners and hammers and things but I don't like taking things apart when I have no idea what I am looking for. I think I will take it to a local (North Wirral) motorbike shop, who coincidentally has an immaculate looking CB400F in at the moment to see if he can cast some light on the problem.

McCabe-Thiele (Ted):
As a complete novice with most of my experience on cars I would suspect the tappets and/or cam chain tension listening to the noise.

I was looking at the parts manual and could not see any clutch rubbers listed though I have seen reference to them before?

K2-K6:
You seem to have all of the normal items covered in how they are set.

To describe it further, if the crankshaft idle speed is not as stable as possible they run oscillations through the primary drive which clatters the clutch and gearbox (effectively they are flywheel mass remote from crankshaft via primary) and sound mutch worse than it really is.
Any significant slack in primary chain helps this amplification. Believe there's a specification in manual to assess chain wear in situ that could help to see just where it is in lifing terms.

Something else to try that may give indication,  set the idle airscrew all to near 3 turns out to judge what happens.  You should also have to bring tickover down on the main adjustment to stop it going too high.
What this is looking for is to see if the idle jets themselves are flowing too much fuel and "hunting" as if partially on choke.
It's also worth checking the identification of the jets as they are veeeery sensitive to accuracy, with experience on here showing how hard it is to get new replacements that can match the original Keihin ones.
In essence,  this is asking the question of carb rebuild just what is inside them now.

cliff7:
I will try weakening idle mixture as recommended and also contact Matt (Harper) to see if he had any problems with idle jets when he rebuilt carbs. Will concentrate on getting idle as smooth as possible before doing anything else. Thanks.

cliff7:
K2-K6 might have a point. even with mixture screws over 3 turns out bike idles and revs with no hesitancy. Too rich? Idle jets? Float level? Will investigate.

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