Topic: cb750 engine chatter

[scknight]

Hi all, I have a 76 CB750k that I got a little while back that I'm trying to get tuned up. So far I've changed the oil, adjusted the valves and the cam chain tensioner. I have a couple questions though. The main one is about chatter from the engine at idle. The bike has 30,000 miles on it and I don't know the complete history of it. I'm not sure what the chatter is because I haven't been around a lot of these bikes so I'm looking for suggestions on how to diagnose what's wrong. I'm fairly sure the carbs need cleaning because it idles really horribly and wants to die but once it warms up it's fine. Carb rebuilding is on my list of things to do. The clutch slips so rebuilding that is also on my list. My last question is that about the engine temperature, it runs pretty hot. Seems like maybe to hot I'm not sure where to even start to fix that problem. I think it needs a new head gasket because it has oil around the head. Would that cause it? Also the exuast gaskets are in pretty bad shape too. I'm fairly sure the oil pressure is good but I haven't checked. So any advice on the chatter and hot engine? Thanks a lot.

[keiths]

The chatter is most likely do to out of sync carbs. But that's the last thing to do. The heat is most likely due to a lean condition. Do a plug chop to see if your running lean or rich. I know a car will also run hot if the timing is off.
First make sure your carbs are clean.
Adjust your valves, set your dwell and timing and then balance the carbs.
Check the old posts for a more complete check list.

[Bodain]

Is it engine noise or clutch chatter. Clutch chatter is par on these units. Pull in the clutch. Does the noise reduce? If it does, it's typical clutch noise.
Oil on engine. Before you suspect the big items look at the little items. My 75 CB 750 apparently had a valve cover leak. I cleaned up the engine and rode it a bit. Fresh clean oil appeared on the upper cooling fins in front. My first thought was valve cover gasket and engine removal. Turned out the oil was leaking from the old seal where the tachometer drive goes into the valve cover. New seal cost about $4.00 from Honda dealer. No more leaks.

The same tach drive oil seal had failed on my CB 550 also, so it seems not uncommon for a 30 year old machine.

You may find rough idle related to noisy clutch. Anytime you have a rough idle. Seek carburator sync...

[Sam Green Racing]

If the carbs are out of sync, the noise will be from your primary chains.

Sam.

[TwoTired]

My last question is that about the engine temperature, it runs pretty hot. Seems like maybe to hot I'm not sure where to even start to fix that problem. I think it needs a new head gasket because it has oil around the head. Would that cause it?

Lean mixtures, poor ignition timing, poor oiling can lead to high engine temps.  However, it is an air cooled motor and needs air movement to cool.  If it is stationary, set up a fan to blow air on it.

The head gasket seals each cylinder.  There are separate o rings to seal the oil galleys between the head and cylinder block, as well as other seals to keep camshaft oil inside the engine.  Head gasket issues will show up in a compression test.  Seals and o rings can leak on a whim.

Cheers,

[Noel]

Let's bear in mind, however, that this may well be a new owner. If he's never had an SOHC, he may be comparing his bike to a modern bike, and not understanding that

A) The bikes take a bit longer to warm up than a modern liquid cooled bike
B) The engines seem to radiate more heat than a modern liquid cooled bike
C) The engines are louder and "clankier" than a modern liquid cooled bike.

If he is in fact a newcomer to the SOHCs, it would be a good experience for him to hear a few other SOHCs starting, warming up, and running prior to deciding he needs major repairs.

IMO, of course.

[scknight]

Thanks for all the advice. I will double check if the noise gets quiter when I pull the clutch in, but I think not. The next thing on my list of things to do is ignition timing so I'll try that and then finally I'll try to synch the carbs. Overall the bike runs pretty decent. The poster who mentioned I was probally used to modern bikes is correct so I am adjusting to trying to listen and understand all the noises of the new bike. It probally would help to hear some more cb750s run so that I could compare. At this point pulling the engine is waaaay down on my list of things I want to do because the bikes to fun to not ride. So I'll just keep plugging away at all the little tune up things. Thanks again for all the advice.

[paulknapp]

G'day all, my '72 750 has an ALARMING loud rattle when idling, but when i pull in the clutch, the rattle reduces  to a healthy, general quiet rustle. It is smooth and quiet on the road. I have been told the noise is..... (by different experts) ...
  (A) cam chain
  (B) primary chain
  (C) clutch rattle
 The noise doesn't appear to be getting any louder,over 10000K's,  so i presume it is only clutch basket rattle!!!
 Cheers
 Pablo