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Knocking sound at idle?

  • Thread starter Thread starter IOWA450T
  • Start date Start date
I

IOWA450T

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This problem is with an '82 750c Honda. The PO told me that he had the carbs totally rebuilt at a dealer last spring and he reinstalled the carbs but never rode the bike after that (he bought a Goldwing while the carbs were off). He said he did get it started but the carbs needed to be synched. I have been able to start the bike but it makes a knocking sound up to about 1400 rpm. #1 & 2 pipes get hot, #3 not as hot, and #4 cold. I assumed #4 wasn't firing so I pulled the plug wire while running and the bike died.
I plan to pull the carbs and clean them and have bought the adaptors to synch the carbs (will be trying the homemade synch tool I see all over the net). I am a little leery about carb work having never done any but I need to get my feet wet sometime. I guess the gist of my ramble is this: could the carb issues account for the knocking at low rpm? If not that, what?
 
I've restored two non-running Honda CB750 bikes (one a 1980, the other a 1981).
I wouldn't check carbs first. That should be checked last.
The low RPM knocking is typical when the difference in compression between cylinders is more than a few percent difference.
The 1980 CB750 I mentioned above had very low compression on one cylinder because of a head gasket leak. Other cylinders had low compression because of insufficient valve clearance. Higher RPM seemed to make things better, so I ignored it for several thousand miles. Eventually the head gasket leak became so bad that the bike would be nearly impossible to start and would not idle below 2000 RPM. That's when I bit the bullet and replaced the head gasket and adjusted the valves to spec (the engine has to come out of the frame for the CB750 head to come off!!!!!). Now, it still knocks minimally for the first 30 seconds after startup, but once it's warm it idles fairly smoothly at about 1100 RPM (not to mention much easier starting, and much improved power).
After I got all that sorted out, it started doing the same thing - this time it was an intermittently failing coil. The borderline coil caused the bike to bog above 7000 RPM. Eventually the coil failed altogether and the bike ran on only two cylinders - that's when it became obvious that the coil was the culprit - I messed with carbs for days - what a waste of time.

So the moral of the story: Verify the easy stuff first.
1. Check compression.
2. swap with "known good" ignition components (coils/igniters/pulse-generators/spark-plugs-and-wires).
If all that checks out OK, then investigate the carbs.
 
Well call me a fool but any time I purchase an old bike with an unknown maintaince history, the first thing I do is perform a full service including valve clearances and carb cleaning. I don't believe in trying to guess on what could be wrong until I KNOW what is right.
 
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