Topic: 78' CB750 Strange Compression/Oil Problems

[brandondavisetc]

I recently purchased a 750 on a gamble. When i got home and cranked it up I had oil spitting out of the exhaust. I did a compression test and found that 1,2,&4 were all around 120 before oil and never went up more than 10/15% after a squirt of oil. However #3 was reading 25 pre oil and 35 after oil. I am also seeing white smoke out of the exhaust, but i'm thinking it is just from the burning oil that is already in the exhaust. I'm not crazy about doing a rebuild since I have never tackled anything like that, so I want to make sure I know what the problem is before I take that plunge. I am brand new to this whole process so any thoughts whether basic or cb expert would be a lot of help! Thanks!

[Black 750K8]

Oil does not burn white. So #3 is 100psi lower than 1 2 4?? Why didn't you check before you bought it? Take the tappet adj. covers off #3 and see if the valves are moving.

[flybox1]

WHITE SMOKE is most likely condensation. 
ride it for a good while and burn it all out. 
check the oil to see if there is mayo(moisture) floating on the top.
complete all of the 3000mi maintenance items before you even think about tearing into anything.

[PeWe]

The cylinders might have got oil inside as conservation..?

I remember when I blew one 836 piston for many years and tears ago...plenty of dense almost white smoke in my mirrors and less power...  Hole in piston crown. That happened in about 170km/hour

[brandondavisetc]

Thanks for all of the input guys! The valves are all moving/working. I have done all of the 3000mi maintenance except for carbs, but I have a rebuild kit on the way. As soon as I get the rebuild kit here I'll put it in and drive it for a bit to see if the white smoke is in fact condensation. As far as the oil goes, i'm still a little concerned/confused about why it would be coming out of the exhaust.

[madmtnmotors]

Oil in the exhaust pipe means you are probably getting blue smoke (oil) instead of white. Oil smoke is a much lighter color than rich fuel smoke (which is black). White smoke would be water or coolant (?!). Oil in the pipes (and blue smoke) would be valve guide (exhaust) or rings. Based on your compression numbers I would bet #3 has a stuck or worn out rings. Worst case, a lean condition could have burnt a hole in the #3 piston, not likely though, since compression in #3 would probably be much worse (zero?).

[brandEn]

If your 78 is an F model it could be an exhaust valve guide. They are known for wearing prematurely.