SOHC.co.uk Forums > CB750

Vent Pipe Smoking

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steff750:
 hi
      I  have this problem with my CB750 sidecar outfit, which I use as a winter hack, so it never really warms up and has I have not used it yet this year, the oil tank still has a bit of gunge around the filler cap and it is well overdue for a oil change anyway, so that will be my first option as it has always puffed out steam from under seat
          I did have to rebuild my first CB750 years ago when I first had it , it had a hard life as one of the previous owners had used it for hill climbs and sprints and had thrashed it  a bit in the 70's , he told me it had snapped the gearbox main shaft . it had a bit of steam and back pressure and it never went away even when the bike was warmed up and it seemed to be down on power . when I did get around to rebuilding the motor about a year later #4 cylinder was well scored because a piston ring had broken up into about twenty pieces and damaged the bore and the piston groove,  I had to bore out the barrels two sizes over (+50) and added + 50 pistons ,so try all the other options first , a compression test should tell you if there is a cylinder down.

Garryk:
Steff,many thanks for your reply and help.
Checked for gunge round my filler cap and there doesn't seem to be any,just hoping a few days out on the road will clear the steam/smoke then i'll know all is well,will be buying a compression tester from Ebay.
Cheers
Garryk

Garryk:
UPDATE
smelt the oil tank and dipstick today and I think I can smell petrol so im guessing petrol is getting by rings.
Its summer here at the moment as you all know,so I was thinking if I change the oil would it be ok to ride the bike until winter when I can change the piston rings ?,I will  still keep my eye on the steam/smoke from the pipe just incase its not petrol im smelling from oil tank.
Your thoughts on the above matter please.
Garryk

Bryanj:
Petrol in the oil is not bad rings but short journeys on choke

Garryk:
Bryanj,Thanks for that info and for putting my mind at rest.
When I first got the bike I was having bad carb trouble which I sorted thanks to members on this forum,what I did notice in my carbs was the choke was half way open all the time till I sorted it,i never needed the choke for cold starts so that's when I knew something was wrong,so im guessing this was when petrol got into oil.
Thanks Again
Respects to everyone on this forum for help they give to others and for sharing their knowledge.
Garryk

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