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where would blow-by smoke appear?

jimminy

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Gonna keep it short, I hats typing on mobile devices, but want to say hi to the forum and start with a question.

New to the cb750 sohc but not totally unmechanical. That said, no mechanic.

Just picked up a 78 sohc and know nothing about it but that the guy I bought from worked on it but wasn't much better than me, also, not very honest.

When the engine gets hot, I get a lot of what seems like exhaust coming from the area of the air filter under the seat at idle. The hotter the engine, the more smoke blows when it steps down to idle. Wondering if this would be where you would expect to see Ring blowby showing up.

Also seems the oil in res. is clean and stays level, but have a gueage showing fairly low 40psi reved. Thinking maybe not pumping oil and

starting to burn what's there. Cant say, and don't know how to check that out the most simple way. Might unscrew the Guage and see if its just muck coming out. Oil in res looks great, probably better than it should with all that smoke.

Thanks!
 
Yes that is where the crank would vent blowby, into the airbox. If oil wasn't pumping the engine would have stopped running by now. They will not live on low oil pressure. First thing I would do would be to check oil pressure with a different guage and see if it is different. Honda spec for oil pressure is 50 to 64 psi at 3000rpm with the oil at 140 degrees. Oil weight has an effect on what pressure will be as does temperature. Oil that also has been diluted by fuel in it will be low on pressure as well as can cause blowby. First thing I would do would try a different guage and maybe change oil and see what the pressure does.
 
Thanks for the info. Well then, maybe it's something simple like a top end rebuild, and with some luck nothing more.

I was watching carefully earlier today and what I see is at 3000 rpm I have around 70psi so possibly it's overfilled. I was concerned because at idle I was seeing less than 10psi at times.

Now I just have to find someone in LA willing to do the work, most places won't touch anything so old! lame.

Thanks again.
 
Factory oil capacity is 7.4 pints. The oil pressure light comes on at about 7 lbs or less. So I would say your oil pressure is acceptable. Have you changed oil in it since you got it? If not I would suggest draining the oil in the tank and the motor and put in some fresh oil at the specified amount and see if the smoking goes away after riding it for a while.
 
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