CB750 smoke

[jam718]

Can anyone give me info about blue smoke out of the tail pipe?  Things to look for, causes, fixes, etc.....This is a 77 CB750 Supersport.
Any help would be appreciated!

Jamman

[Re-run]

Most likely your valve guides are shot. it was a common issue on the 77/78 F bikes.
The other main cause is worn rings.

An easy test is to get a compression gauge and test each cylinder. After you get the base reading, add about a tablespoon of oil into the cylinders and test again. If the reading goes up for a bit, then it is rings. if there is no change, then it is valves. most likely.

[jam718]

Someone in another post said that if you have it choked too much and running rich it will cause that smoke too...any thoughts?

[Re-run]

Not blue smoke it wont. Too much fuel will come out looking black from being unburned. Oil burns blue.

[wvhardtail]

Had the same issue with mine and it was the rings.

[jam718]

That's what another friend of mine said......are there any additives to be used with the oil to stetch things out or am I looking at a rebuild?  Any ballpark idea what it costs to have a shop do this?  I don't have a lot of extra cash....

Thanks ahead of time!

[wvhardtail]

When I pulled my heads I found that my cylinders were fine. All I had to do was hone the cylinders and put in new rings. Not that big of deal really. If they are in bad shape you may have to have them machined. Unfortunately you won't know unless you pull them. There are additives that you can put, ie. Motor Honey, but I would not try to stretch it out too long. You can burn up your valves and cause a lot of carbon build up.

[Re-run]

Yeah not only that but who knows what that stuff would do to the clutch.

[wvhardtail]

Good point re-run. I am used to working on Harley's with seperate gear box.