I have a 1975 DT 175B rebuilt engine. I'm fairly certain it's burning oil. It smokes... but not always, after some time, there is less oil, and the spark plug seems to get fouled easily. I replaced the right seal again and think everything is good there so the only thing I can think remains would be a leak through the crankcase halves through the Yamabond. I was pretty clean with the Yamabond - not too much, not too little.

Anyway, my question is could I detect a leak like this using a leak down pressure test? Or could it be comming from somewhere else?

Jeff

Yes, a pressure test can find the leak. 

What I do is make sure trans filler cap is tight and stick a golf tee or something into trans vent hose.  (Clutch cover on)  Pressurize engine to 5 psi...  spray soapy water and fix the engine leaks you can find.  (Don't forget tester connections).  If it's leaking and you can't find the source in the "engine" part...  then start spraying soapy water around kick start, cover gasket, shifter seal (pain on that model as you need to remove linkage cover), etc.  If you see bubbles it means pressure is getting into transmission.  Or maybe trans is sealing just fine...  then with pressure still at/near 5psi in engine, remove the golf tee from trans vent.  (best if quiet.)  If you hear a "whoosh", then pressure is getting into trans.  You can also put your thumb over vent hose for a couple minutes while maintaining 5psi in engine you should be able to feel pressure escape when removing thumb.  Or submerge trans vent hose into a cup of water and it will blow bubbles. 

If pressure is confirmed leaking into trans, then you need to remove clutch cover and check crank seal and areas of case you can get to with soapy water.  If you can't find a leak there, it's likely the case halves.

I always vacuum test after pressure testing and getting all the pressure leaks stopped.  It is possible to have the engine seal perfectly under pressure where it takes hours to lose even a fraction of a psi...  but not seal at all under vacuum. 

I know that's a lot and might not make sense if you haven't ever done it...  post your progress and results and people here will try to help.

Just my two cents worth. How does it perform.
Is the exhaust bunged up.
If so, you end up using more throttle than normal which also gives more oil pump oil but not burning it all as it should.
Is trans oil disappearing.
The exhaust starts smouldering within & can make things look far worse than it really is.

how do you know its burning too much oil?
does it run good?
is transmission oil level going down?

Thanks all. First time after the rebuild, it ran just ok but i would keep having to replace spark plugs. I even went a pilot jet smaller. But spark plugs still seemed to get fouled and oily. After about a total of one or two hours on it after rebuild, I noticed that about 50% or more of the oil was gone. Then I replaced the right seal just to be sure but after starting it, it seemed to definitely smoke a lot. The seal replacement went well. I can't imagine two bad ones.

I guess I'll try a pressure test starting with testing for air out of vent tube and then at the right seal or anywhere in that area with the water method.

I could see how you may have a valve like action with the Yamabond gasket which may result in pressure loss with a vacuum instead of positive pressure. If this is the case, I assume this would be the case halves? I can't think if any other way gear oil would get in to the combustion cylinder.

Thanks so much for the feedback.

Actually if it's the case halves not sealing you almost certainly find it with a pressure test.  Crank seals are "dual lip" to seal against pressure and vacuum and sometimes they'll do one and not the other. 

 

Ok. I just ordered a pressure tester. The seal is double lipped for sure. I assume if it's the case halves, then I would see bubbles in the clutch chamber where the halves meet.

Thanks!