Honda-SOHC
SOHC.co.uk Forums => CB350/400 => Topic started by: cliff7 on June 10, 2021, 12:52:05 PM
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I wonder if anyone can help identifying a noise on my bike. I have had the clutch apart and everything seems in order (to me) and clutch rubbers seem to have been replaced with no obvious back and forth play.
https://youtu.be/87jPjI33DMw
Engine was supposedly rebuilt not too many miles ago. Don't know whether I have done the YouTube link right.
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Certainly it doesn't sound as refined as it should, the sound of the engine will be amplified through the transmission and with the clutch pulled that's normal to hear less noise.
More to the point is why it's noisy in the first place. Rebuild can mean such a different level of internal condition on these engines with no real easy way to diagnose, certainly from a remote look most view is speculation or guesswork.
Initially it would be wise to verify general service items, tappets, ignition setup, carb sync, cam chain tension etc to make sure it's not contributing to the sound.
The immediate suspects for inspection are the cam chain tensioner and if that's working as it should, then looking with the sump removed to see what level of wear exists in the primary drive chain. With any significant wear in either chain then you'd not expect it to run smoothly.
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It's so difficult to tell from a phone vid but it doesn't sound right. How long ago in miles and time was the engine rebuilt, what did the rebuild consist of? What does it sound like when you rev her up?. Are you 100% sure the clutch rubbers have been changed? It sounds very 'top end' are the tappets adjusted correctly?
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Many thanks for replies. Previous owner had it for about 2 years but hardly used it. Owner before him passed on loads of invoices and notes but not a proper diary. it goes quieter when revved. Clutch rubbers have screws holding them rather than rivets and I can't get any movement by hand. Tappets have been done and checked again. Timing seems spot on. Horseshoe is free as a rod down blanking screw hole moves up when rotating engine backwards. Had sump off but not sure how to check primary chain. Oil light goes out immediately on start up. Wish I lived closer to those that might know. Thanks again. Oh, and noise doesn't seem louder with dipstick removed.
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When were the carbs last balanced, out of sync carbs are the biggest cause of bottom end clatters on these old SOHCs
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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.
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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?
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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.
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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.
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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.