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Oil spring on C90

Altmuehltaler

Altmuehltaler

Registered User
Germany
I have a little oil spring under the cowling and can't find where its origin is.
The engine is very clean. Two cylinders sweat a bit at their base. I think oil consumption is OK, too (less than 1qt per 10h).

It looks like the oil comes from the bearings which support the shaft of the carburetor heat valve in the carburetor air box.
On the inside of the air box just behind the hole through which the unused carb heat air exits I always find a puddle of oil in the corners (refer to pictures).

After each flight the lower side of the air box is oily and from there downstream the cowling and belly are greasy.

The top of the air box is dry.

I have no idea where the oil comes from. :???: Is there anybody out there having an idea?

Thank you!
Bjorn

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Take a white rag/T-shirt and rip it up into narrow strips. Clean the engine and then tie the strips to all the tops and bottoms of all the pushrod tubes. Go fly for 10 minutes and check the rags.

Glenn
 
The oil might be coming from the inside of the intake tubes, down through the spider, down through the carburetor throat and into the airbox.
 
cpinckney1 said:
The oil might be coming from the inside of the intake tubes, down through the spider, down through the carburetor throat and into the airbox.
Click to expand...

Interesting. I had this idea too but discarded it because I couldn’t imagine oil flowing down the intake tubes while air and fuel are rushing up... Maybe I was a bit hasty refusing this idea.

If your idea is the case where could the oil come from? Worn out valve guides?
 
You can speculate forever. Knot the rags around anything you suspect and find a place to start.

Glenn
 
Clean engine with solvent, dry it thoroughly. Thanked baby powder and lightly coat the entire engine. Run 5 minutes and look for wet spots on the baby powder. Run 5 minutes, repeat until you get the first indication of an oil weep. Fix that and repeat until you don’t find any more leaks.


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If examining the engine externally fails to find leaks, then remove the valve covers and examine the intake valves. Pay particular attention to any cracks in the cylinder head plus valve guide-stem wear, and also the type of rocker arm installed that activates the intake valve. Continental designed those arms for the exhaust valve to pass oil though them internally to add more lube. Sometimes they are also installed on the intake valves. I suppose it's possible for a combination of excessive oil and loose working clearances to allow oil to eventually drain down the inside of the air intake tubes into the air box.

Edit: Also during closed throttle operation high vacuum in the air induction system can draw oil around the valve stem and guide if their clearances are excessive.

Gary
 
Last edited:
Had to trace one one time. Dad pulled cowl, cleaned engine with solvent, then i went and ran it up hard as the brakes would hold and as long as my leg would hold. Then shut it down. Was a through bolt leaking on the left front cylender. Most intake and push rod tubes are easy to see for some reason, but that through bolt was impossible to find if you flew it.
 
One quart every ten hours and a bit of oil on the carb heat shaft?

While I strongly subscribe to dga's method, in your case I would spend more of my days flying and my evenings sipping beer. Don't get all carried away - these are lawn mower engines, and they just leak a bit here and there. Check your compression daily (always check mags and throttle first) and when you get a weak cylinder, watch it for a couple weeks before you pull it.

I have six thousand hours behind small Continentals - they seem bulletproof as long as you stay away from folks with red tags. My only actual failure was my fault - I let a non-FAA shop balance a C-85. Never do that.
 
Bob, thank you for your reassuring comment.:)
Concerning performance there is absolutely no problem. Takeoff performance is good and differential pressure tests are also good.

Yesterday I flow an hour and then removed the intake tubes. I found a bit of oil in all of them (#1 and #3 a bit more than #2 and #4). So this seems to be the source.
The engine has about 600h since last overhaul which was in the mid 1980s. Only cylinder #4 has been changed recently (60h) and to find oil in its intake tube disappoints me a bit...

Anyway, I think I will / have to live with it as long as there is no performance problem. And I will have some beer... :Beer:Beer:Beer
 
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