SOHC.co.uk Forums > CB350/400

400 four knocking noise.

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UKROBK7:
Did you check the bearing shell clearances with Plastigauge (gage) when you had the engine down to check the clearance against the Honda spec?  A couple of my shells on the 750 looked great but they were down below tolerance.

Lynx:
Stevie,

What was the outcome of this? I have a very similar problem on my newly rebuilt engine.

Cheers
Steve

Dan:
I would put money on this being carbs out of balance. I had exactly the same problem also. The first time I tried balancing the carbs using vacuum gauges I got nowhere.
I cured this by removing the carbs and setting the carb slides as best I could by eye, then refit carbs and make fine adjustments with the gauges. Result - knocking gone and fears of bearing/primary chain problems allayed.

Search this on You Tube "cb400 4 carb synchronisation"

Dan

Lobo:
My newly rebuilt engine too has this knocking in its boots; and I'm at a loss at to whether its 'normal' or otherwise. It was professionally rebuilt, inc new can chain, tho' not primary... which I'm told is fine. I've balanced the carbs pretty bloody spot on (I think!)... the only query here is they pull only 16"Hg @ idle versus the typical book figure of 20-24". Comments on this? (My '72 750 same & runs fine)
The knocking is a little alarming @ 900rpm which is rather slow I guess, by 1200rpm the knocking is 'ok', and by 1400rpm the motor's just silky smooth. Due bike not registered unable to give it a true shake-down, but don't imagine this knocking will mysteriously disappear.
Would really appreciate a definitive answer, its had new clutch basket & springs.
Not truly alarming... Its just that the rest of the bike is so good!

Lynx:
Well I'm pretty sure in my case it is carb related as carb 2 still doesn't seem to be working. Looking as old posts this seems to be a common problem on new rebuilds. If I raise the idle above 2k it souns OK but very little throttle response. Below 2k it sounds like the engine is about to break in half.

Spark plug, cap, lead, coil all tested OK. Compression fine. So I've stripped the bottom end of the carbs again, cleaned jets and passages with Wynns carb cleaner and blown through with compressed air. Re-installed but no change at all :'(. Tonight I'll try replacing the pilot fuel jet, fuel valve and float and cleaning again.

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