Smoking

[DuoDave]

Hi folks, just joined the site. My early model CB750 smokes badly under load from the exhausts and engine breather. The cylinders have been rebored to +25 and has pattern pistons (ART?) and genuine honda rings. The bores look good, the pistons are a close fit and the ring gap is within tolerance. The rings were fitted with the N25 stamping facing up and spaced with the gaps at 120 degrees. I tried fitting new valve guide seals, but that did not help. The next step will be to take the valves out and check the guides. One thing I noticed is that there are two types of scraper ring in each set. One is stepped on the underside and the other is plain. There is no mention in the workshop manual of which grooves to fit them in. I don't see that it would make much difference anyway, but I would appreciate any info on this, or the smoking problem generally.

[Spitfire]

Hi Dave,
             It does make a lot of difference where you put the different types of rings, as they are designed to do different jobs (sealing etc)
The top compression ring should be marked (R,T etc) and if you look closely it will be concave at the top (and shiny), the middle oil control ring has a step in it, the step should be at the bottom (piston skirt).
The oil scraper ring comes in three pieces (old ones are one piece), two thin rings and a wavy ring (technical term !), again these two thin rings should have their gaps staggered.
Also ensure that the piston ring grooves are clean and that the rings can move freely, measure the clearance with the ring in place, max 0.18mm for the top ring, 0.165mm for the second ring.
If the breather from the cam cover is blowing then your crankcases are pressured up indicating piston/barrel problems.

Cheers

Den

[SteveD CB500K0]

Good reply on the rings Den, but just had a thought.

Is the smoke blue/white or black?

Blue/white means burning oil but black smoke could be just very rich running.

Have you done a plug chop? If so what were the plugs like?

[DuoDave]

Thanks for the replies. I had the stepped ring in the top groove, but I don't think that would be the whole problem. The smoke is blue and the oil level in the tank drops noticably after a long run. The smoking is most apparent when the motor is under heavy load, like pulling hard uphill. Piston ring/bore problems usually show up by a puff of smoke when you shut the throttle and the resultant vacuum in the bore sucks oil up from the crankcase. I don't see that with my bike, so I suspect a valve guide problem.