Smokey 750

[ianmck]

Any ideas, thoughts on my smokey 750. Bought Nov 08, been laid up up for quite a long time after having been imported from America, it started and ran quite well, sounded pretty good and no smoke. Ran it up at home , and had a couple of small runs on it, before starting to tidy it up, once again, ran quite well, and no smoke, did a few jobs on it, and didnt run it for a few weeks, started it up in the garage, and as it warmed up I noticed some blue smoke from the exhausts,allfour were smoking, but cyls 3 & 4 were quite bad. Changed oil and filter, cleaned pick up gauze, re filled with oil, still the same, put a mix of redex and eng flush into bores,left to soak 24 hrs, drained oil cleaned filter,re filled with fresh oil,still the same. read all the posts about blue smoke, removed engine to check valve seals and rings, all looks to be fine , engine is almost spotless inside, and no sign of any undue wear and tear, HELP with any ideas, suggestions as to any other possible causes, would be very welcome.  Ive thought of oil press release valve stuck/faulty, blocked oilways, cant think of anything else, I am distraught,
Regards ianmck.

[Seamus]

Still don't know much about the engines on these, but could it be posible that a breather pipe has become trapped? Surprised that it just developed.
When you had the engine apart, did you check the valve stem seals?
Also I have known glazed bores to cause this problem on other engines, but again, I suspect that this would happen gradually.

Any use??

Seamus

[Bryanj]

valve stem seals

[Spitfire]

It seems strange that it ran OK and didn't smoke until you laid it up for a couple of weeks, did you do any work on it in this time ! Which CB750 is it, is it a later one with the recycling breather system or an early one with just a breather pipe from the rocker box.
I would backtrack on the jobs that you did on the bike in the period from going from non smoking to smoking.
Does the smoke clear when you take it out for a run ?
I would certainly check these are clear, and check the carbs, timing etc, but at the end of the day Bryan will probably be right.

Cheers

Den

[ianmck]

A friend of mine who has a car engine reconditioning business took a look at the engine and finely honed the bores and found that they were quite worn and possibly ovalled, the outcome being is a re bore, new pistons and rings, and the head overhauled, including valve stem oil seals. but he could not explain the no smoke then smoking mystery.
Anyway thanks for the help, and keep fingers crossed that this will cure it. Will let you know.     Regards ianmck.

[Bryanj]

Tell you what, if the bores were worn and oval that motor has been abused and run with no aircleaner, I can remember a 120,000 miler where even the ring gaps were still in tolerance!!!

[ianmck]

Hello Bryanj, I think you might be right, lets just hope this sorts the problem and I can enjoy the bike come spring/summer, fingers crossed. Regards ianmck.

[daveh]

Just a thought if the engine is wet sumping this can cause excessive smoking on start up until the oil pump scavenges the sump.
regards
Daveh