JD318/Small Engine Experts?

[timb]

How much is too much compression on a small air-cooled engine? Anyone familiar with the idiosyncrasies of an Onan?

Last year I started pulling a lot of oil into my 87 318's carb (P218 Onan) - and it *seemed* to be coming from the breather/crankcase (and not from an external oil leak via the "snorkel". So I parked it until I got a roundtuit to see what was up.

Finally got that roundtuit - so I ran a compression check over the weekend. Figured if I got good numbers there - then the oil-through-the-breather was *probably* not a bad ring(s) or gasket causing excess crankcase *pressure* but a bad breather valve. I got 124 psi in both cylinders, engine warm, with the throttle and choke plates wide open.

Sounded good to me - but for what it's worth that is actually a higher reading than I've been able to find anyone posting for a 318. The tech manual just lists a "minimum" reading of 75 psi.

With those numbers - and the consistency between the cylinders (both the same) I packed everything away and forgot to run another test with oil in the cylinders. I ordered the breather parts and will install them next weekend to see what that accomplishes.

I know the Onan's need periodic de-carboning - but I haven't had the heads off yet. Anyone have an opinion on the compression readings and / or the likelihood of the breather being the oil culprit?

 

[mwood]

I think you're on the right track with the breather. It does seem to be very high numbers on your compression test. Though high is good, too high can be bad. It could indicate that you have excessive carbon build up or something else wrong with valves, etc...

While you're waiting for you parts, I'd pull the head and clean it off really good. Also, pull the muffler and check to see if there is some major carbon buildup in the exhaust piping. It can't hurt but probably won't fix your problem.

I'd say the breather is your culprit.

Mark