Topic: Transmission Whine 1974 CB750

[NewOldSchool]

Hello all,


Iv'e owned my 1974 CB750 for almost 7 years. It was purchased with 16,000 miles on it and now I have about 58,000 on it. I have followed the oil change and maintenance schedule religiously and always used 20w50 Honda Oil. Full tune up every 2000 miles. I have rebuilt the top end, new rings, cam chain, valve guides, 3 angle valve job, seats, on and on. Every bearing, bushing, gear and seal has been touched........

 except the bottom end of the engine.

In the last 1000 miles it has developed a whine in the transmission. It only happens with the clutch let out and the pitch and volume seems to increase slightly with load and rpm. It will also make the noise in neutral with the clutch let out, but not with it pulled in. My thoughts are that the straight cut gears have finally worn to a point where they make this noise and it will be something to live with, or that a bearing or trans shaft bushing is/has worn past serviceable tolerance. There are no metal shavings in the oil and there is no excessive drag in the transmission. Any thoughts? Bite the bullet and do the bottom end? I'm just bummed as I've heard plenty of times the bottom end on these bikes are good to 100k miles or slightly more. Thanks! 

[flybox1]

It only happens with the clutch let out and the pitch and volume seems to increase slightly with load and rpm. It will also make the noise in neutral with the clutch let out, but not with it pulled in.

Clutch plates.  How old are they?
Go there first. 
Much cheaper and less involved than pulling the engine

[NewOldSchool]

It only happens with the clutch let out and the pitch and volume seems to increase slightly with load and rpm. It will also make the noise in neutral with the clutch let out, but not with it pulled in.

Clutch plates.  How old are they?
Go there first. 
Much cheaper and less involved than pulling the engine

About 10k on the fibers and steels. Its definitely gear speed related, coasting to a stop in gear with the clutch pulled in, it still makes the whine, if not worse than in neutral at a stop. Thanks for the suggestion!

[nobody]

If it makes you feel better, I had to rebuild my transmission with only 19k on it.

Bad shift in a drag race... pop!


I'd check your clutch like flybox said. If no dice after that, just tear it out and redo it all, it'll be brand new when it's back in the frame. My guess would be bearings.

[NewOldSchool]

If it makes you feel better, I had to rebuild my transmission with only 19k on it.

Bad shift in a drag race... pop!


I'd check your clutch like flybox said. If no dice after that, just tear it out and redo it all, it'll be brand new when it's back in the frame. My guess would be bearings.

Does sound like a good place to start, thank you!

[NewOldSchool]

Any other suggestions?

[nobody]

With the trans, if you can't figure it out from the outside, you're almost just going to have to get in there and see what's broken.

[nobody]

If you end up doing the bottom end, this thread might help:


http://forums.sohc4.net/index.php/topic,59158.0.html

A few of us were all doing trans rebuilds at the same time, lots of helpful stuff in there.

[chewbacca5000]

I would be checking that clutch and clutch basket before you do anything else.  I know it is a pain to pull the motor, but taking them apart is easy.  Just make sure to get all the bolts out, tap the case a bit, then find the 2 pry points on the front of the case.

Use a flat head scewdriver and a hammer.  Tap, tap and she will open up for ya.

Many have driven lots of miles and most engine noise is either the tappets making their sowing machine sound, or the clutch.

[NewOldSchool]

Thanks all, yea I've had the motor out twice already in the past. I wasn't looking forward to another weekend long job. Time to face the music!

[CB750F2]

Even before removing the clutch for inspection I would check that you have the clutch cable adjusted correctly to ensure that you have no pressure on the center of  clutch release bearing when the clutch lever is released. If you find adjustment was required and the noise goes away then dis-assemble the clutch and check the release bearing and clutch. Pat

[CB750F2]

Also, while you have the clutch removed start up the bike and see if you have oil coming out of the center of the transmission shaft. If you don't have oil coming out of the end of the shaft with the motor running you have a lubrication problem and may be the cause of internal transmission bearing/gear failure. Pat

[NewOldSchool]

Also, while you have the clutch removed start up the bike and see if you have oil coming out of the center of the transmission shaft. If you don't have oil coming out of the end of the shaft with the motor running you have a lubrication problem and may be the cause of internal transmission bearing/gear failure. Pat

Sorry, meant to ask the question assuming I had serviced the bike and all other adjustments are within spec. Thanks!