Topic: CB750 Primary chain

[kaysystems]

I've just taken the engine out to do a valve job.

I checked the primary chains. They seem a bit stretched (not much movement left on the tensioner). One chain seems a lot looser than the other. Is this normal?

Guess I should replace them while I can.

Thanks

 David

[MRieck]

Yes, it is common. I'd replace them and the tensioner.

[ofreen]

Out of curiousity, how many miles on the engine?

Greg
'75 CB750F

[kaysystems]

36,000 miles

I know that the rear chain came off, and messed things up a bit, perhaps locking the engine.

David

[Kevin D]

My primary chains - and cam chain too - have about 30000 miles on them. The tensioner wheels are original. Anybody know the life
 
of these things?? I don't think it would be pretty if any of them broke. I changed them about 25 yrs ago as a preventative

measure but these days I'm not such an enthusiastic mechanic as many of you.

[ofreen]

The tensioner wheels are original. Anybody know the life of these things?? I don't think it would be pretty if any of them broke.

That's a good question.  My 750 has 99,550 miles on it today, with no work ever needed to be done on the engine.  I've had it since it had 3900 miles on it.  Not only do those tensioner wheels and pads have a lot of miles on them, but they are over 30 years old with innumerable heat and cooling cycles.  I'd hate to tear the engine down since it runs as well as it ever did and uses almost no oil.  But a catastrophic failure would be a sad thing.  What to do, what to do.

Greg
'75 CB750F

[MRieck]

I hate to say it but every rubber piece in your engine subjected to oil and heat are no longer "rubber". They are cooked. Reach 100,000 miles and tear it down. You'd be surprised how much better it can run.

[ofreen]

I hate to say it but every rubber piece in your engine subjected to oil and heat are no longer "rubber". They are cooked. Reach 100,000 miles and tear it down. You'd be surprised how much better it can run.

Of course, none of those things were ever made of rubber. They are plastic, apparently a rather tough plastic.  The engine still runs strong, gas mileage is in the low forties, I never have to add oil between changes at 3000-4000 miles.  I haven't had to adjust tappet clearances in years, they don't change.  The biggest thing I have had to do to it was replace the seals under the cam towers a while back.  I don't baby the bike, it still sees redline occasionally,  but it has been well cared for.  It has been ridden in temps from 0F thru 110+.
   
I have heard of these engines going 150,000+, although there probably aren't a lot of those out there these days.  Is there anyone on the forums that have any 750s over 100K and have done a teardown?  I'd be interested to hear what they found.

Thanks for the advice.  I'll keep riding it for the time being and keep an eye on things.

Greg
'75 CB750F