O-300A Oil Leak from Carb

[ptporebski]

Hello,

I have an O-300A with about 250 TSMOH (2004) that is dripping oil thru the carb, specifically from the carb heat opening on the air box. After flight I get about a teaspoonful of oil leakage. I have also noted that unless I very aggresively lean the mixture during taxi I typically have fouled plugs. My AI says although all cylinders were 79 at last annual (15 hrs ago) there is probably a valve stem bushing leaking by. He told me to pull the bottom plugs this weekend and that one should be "wet". I pulled the top plugs yesterday, but they were all dry. The AI also said if all plugs were dry, the next item to check would be were the induction tube goes thru the oil sump, but he rather doubts that is the issue. We also looked at the carb - but he says that the carb is not leaking.

Does anyone out there have any other ideas as to what this could be?

[lowNslow]

This is also the area where the oil sump will show leakage if is corroding through, very common in the O-300 with the magnesium sump. The sump will usually corrode through just forward of the carburetor pad where water collects in the sump.

[edbooth]

This is also the area where the oil sump will show leakage if is corroding through, very common in the O-300 with the magnesium sump. The sump will usually corrode through just forward of the carburetor pad where water collects in the sump.

This is true, but does not address the problem of oil leaking out of the carb heat outlet in the bottom of the airbox. Will have to think on this for a bit.

[jrenwick]

I've seen this when there's a little too much clearance between the intake valve stems and the guides. It could be more than one cylinder responsible. It won't show up on the plugs. I think the maximum tolerance here is supposed to be .005", which might be enough to permit some leakage. Oil from the area under the valve cover travels down the intake valve stem to the intake port, and then down the manifold tubes to the carburetor. The oil that drips out the carb will be tinted with dye from the fuel.

[GAHorn]

Your a&p is correct....you have valve stem/guide issues and oil is leaking past your guides and down the induction tubes, into the carb and then into the air box. It may be worn guides or it may be a broken guide.

[blueldr]

Put a drip pan under it after shut down to keep the floor clean and fly it. I always figure that if there's enough oil in it to last through the fuel supply it must be OK.

[GAHorn]

I've seen this when there's a little too much clearance between the intake valve stems and the guides. It could be more than one cylinder responsible. It won't show up on the plugs. I think the maximum tolerance here is supposed to be .005", which might be enough to permit some leakage. Oil from the area under the valve cover travels down the intake valve stem to the intake port, and then down the manifold tubes to the carburetor. The oil that drips out the carb will be tinted with dye from the fuel.

Not disputin' that ALL the symptoms mentioned will, or will not, be present... but I've seen this problem on about a dozen airplanes and none of them demonstrated fuel-dye in the oil (but, like I said, that ain't sayin' it can't happen.... it's just that the lack of dye in the oil doesn't disqualify this diagnosis of loose valve-stem/guides)... nor does the lack of fouled plugs. In the cases I've witessed... lower oil-fouled plugs were common.
A solution to the oil-fouled lower plugs can be found in switching those to fine-wire plugs...and the engine might operate satisfactorily for a long time thusly... but it sounds as if this particular engine has a pretty-bad case of it, seeing as it's drippin' so much thru the carb.

My 206 once did this and it turned out to be, not worn guides, ... but ONE BROKEN guide in a single cylinder. That engine was running pretty good but every so often it would get a wee-bit rough for a few minutes ...then straighten out again... doubtlessly due to that valve not seating completely/squarely because it was not "guided" properly onto it's seat. Oil would drip onto the ramp from the induction-system. Changing the guide fixed it.
My Baron did it on it's right engine, and changing the culprit cyls/guides fixed it. Until I ponied up with the money to top that engine, fine-wire plugs got me by for another couple hundred hours (by which time I'd gotten enough money from leasing it out to pay for the top overhaul.)

It's very unlikely that this leak, as described, is a sump problem. That problem doesn't usually leak oil into/thru the carb.... it does it directly from the sump-floor just forward of the carb-base....(although I suppose in some rare situation it could corrode thru the induction-wall, leak into the induction-passage, I sincerely doubt that's what's occuring. My bet...using ptporebski's money....is it's vavles/guides.)

ptporebski: When your engine was "overhauled",... was it really and truly? Or was it a "repair", re-using many parts.... perhaps even utilizing rebuilt or repaired cylinders?

[blueldr]

If oil sump corrosion opened a hole from the sump through into the induction passage large enough to show up as an oil leak into the air box, that SOB would suck the sump empty "toot sweet" and smoke like hell at idle under high vacuum. When that oil got hot and thin and the throttle was closed on a let down, it would look like a sky writer.

[GAHorn]

If oil sump corrosion opened a hole from the sump through into the induction passage large enough to show up as an oil leak into the air box, that SOB would suck the sump empty toot sweet and smoke like hell at idle under high vacuum. When that oil got hot and thin and the throttle was closed on a let down, it would look like a sky writer.

Awww....now you've gone and gotten all technical and confusing us! Use plain ENGLISH,...can't you?

(That's a pretty good analysis! I might save that in the MX LIBRARY,...no kidding.)

[blueldr]

George,

You can go back to watching the video of the pole dancer now!

[GAHorn]

George,

You can go back to watching the video of the pole dancer now!

Glad to know you received my technical video. I still can't figure out how one gets up that polished pole, backwards and upside down with those patent-vinyl boots! That must be a new procedure young mechanics learn these days...

[blueldr]

Yes, of course.

[ptporebski]

My engine was overhauled in 2004 using six new Millenium Cyl's. It was done by a previous owner and I have receipts/yellow tags/etc in the plane's records totaling about $22k. They didn't cut corners as best I could see. Overhaul was done by an AI at the field where the plane was long time based. The AI has a good reputation for doing conscientious work.

Blueldr is correct. I forgot to note that the oil is dyed blue. In fact, I was at first thinking that I have a slow fuel leak. I have asked the AI to look at the plugs today as I flew the plane for an hour yesterday.

I can live with the oil leak for a couple months while I save up for a reworked cyl - as long as it is SAFE to do so. I'm still recovering from buying a new prop ($3,700!!!) I am getting a full lesson in the joys of aircraft ownership. I expected to have a couple of years of finding things like this. But I will be happy when I get the plane up to snuff.