I've had my CB750 for a year now; until recently had overhauled everything except the engine. The engine had poor compression in cylinder 2 (50 vs 150 in the other three cylinders) and was blowing smoke from cyls 3 and 4 when revving over 3000rpm. A friend of mine who has overhauled several bikes helped me pull the engine out recently and I've been helping him to overhaul it.
We have now diagnosed the problems and I thought I would post our findings here.
Cylinder 1 - all good; rings in spec (just).
Cylinder 2 - blackened piston crown and evidence of corrosion. It seems that at some stage this cylinder has had water in it resulting in pitting and corrosion of the spark plug thread. Rings way out of spec. We are now going to replace the spark plug thread and have cleaned up the other pitting. The ring gap and leaky spark plug thread appear to be the source of the poor compression
Cylinders 3 and 4 - blocked oil drain holes in the piston. These appear to have been the cause of the oil consumption as the oil has been unable to return down the inside of the pistons. Rings in spec.
Inlet valves were in spec, outlet valves were borderline so we've had the head overhauled and all valves are now 0.0005. We're fitting new exhaust valves and new rings, new seals, timing chain and pulleys while we have it all apart. Cylinder bores and pistons were all well within spec so no need to replace the pistons with oversize ones.
The bike has done 75000km so its good to see that, apart from damage done by moisture and the passage of time, its in good shape.
Any comments by others on this forum are welcome. I know I have searched long and hard to find the possible causes of the oil consumption and had been convinced it was valve guides; not the seals as there was no oil smoke on startup. My bike mechanic thought it was rings. It now seems that both valve guides and rings were in spec so it appears that the blocked oil drain holes were the culprits.
