Partial power loss

[mikeacro]

I recently purchased a ‘46 C-140 with the C85. Flew it 2 hrs to my airport uneventfully. The next day I went to put some more time in it and shortly after take off I experienced a partial (150-200 rpm) loss. I had about half fuel on both sides. My climb rate went to less than 100 fpm. I switched from the right rank to the left, and everything returned to normal. I climbed to a safe altitude over the airport and went back to the right tank, and the problem returned almost immediately. I I also tried carb heat, as well as playing with the mixture…..no change. Back to the left tank again and all was well. I set up 2400 rpm cruise power at 4500’ and and switched back to the right tank, and all was well. But if I went to a full throttle climb, the power loss returned. After landing, I verified that I do have the proper fully vented caps on both tanks. I’m thinking the restriction may be coming from the selector valve. Thoughts?

[6643]

There is a strainer in the tank outlet in the wing root. I think I'd start there. Also, if you can get a camera or "endoscope" inside the tank it's worthwhile to take a look. Disconnect the line at the tank and carb and blow it out.

It's possible that the handle and shaft are not properly attached to the plug in the valve and you're not getting full motion, although I would expect it to affect both tanks. If you take the cap off the valve (the packing nut) you should be able to tell if it is moving correctly, but I'd drain the tanks first. You could get lucky and be able to see without leaking a lot of gas, but if the plug gets loose in the body things will go down hill rapidly.

[6597]

FInger strainers in tanks

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Fuel selector pics, they aren't difficult to work on but John C makes a point about the handle being loose a possibility.

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Check to see if you have the old black plug (Jensen) retrofit in your fuel selector. Randy Thompson has a story about his failing. The original Weatherheads had a brass truncated cone that ran against a brass body. When properly lubricated (fuel lube) they worked but would gall eventually. Jensen came up with a polymer plug that self-lubricated which sounded great but apparently they either got brittle or failed somehow. Easy to check, they were black.

[6643]

Jensen came up with a polymer plug that self-lubricated which sounded great but apparently they either got brittle or failed somehow. Easy to check, they were black.

The reported problem with the Jensen plug is that the area where the shaft is pinned to the plug would give way and the plug was stuck in whatever position for the duration of the flight (or glide). They suddenly disappeared off the market. I'm surprised there wasn't an AD issued. I don't know if it was a problem with the part, or if they were being damaged during the installation process.

I recall they were made of Delrin. I though they were white, but that may have been me projecting my experience with another (white) Delrin part. I never actually saw one.

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[6597]

The Weatherhead/Anderson 6749 has a white delrin plug

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[6863]

Regarding lubrication of fuel system components, the ag pilot from whom I bought my airplane is a believer in mixing 4 ounces of Marvel Mystery Oil with 10 gallons of 100LL for upper engine lube as well as other fuel system components such as sector valves and mechanical fuel indicators. I continue to do so regularly. This may be just myth and no more than myth. The older i get, the more I believe in myth. This myth along with placing a plastic 5 gallon bucket underneath my tailwheel at night to keep the smallest mice out has worked for me since 2006. Fingers crossed.

[6643]

The bucket thing is not a myth...

[4004]

Just a thought - when all above have failed, you may have a problem with the float in your carb, worn needle and seat, shaft, etc. with the difference in the "head pressure" between the two tanks, being just enough to change the carb internals. Kinda a long shot! 2c PS Be sure to post your resolution, you've got a strange one!!

[6643]

I verified that I do have the proper fully vented caps on both tanks.

One more thing...

I'm not sure how it would be an issue, but, do you by chance have the tube in the front of the tanks that connects the two air spaces? You should be able to see it (or not) if you remove the forward wing root fairing.

[6597]

What John is referring to, this is the tank inter-connect on a 170A but it's the same idea, and much like the inter-connect on a 140A model. The later 120-140's that had the inter-connect I don't believe had the central breather on the top of the cabin.

I've seen several iterations of fuel system bastardizations that the mechanics clearly didn't understand how things worked.

I highly recommend if you haven't already to read Neal's articles on the fuel systems. Correct fuel caps are a must, a lot of shade tree stuff out there.
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When I first got my bird 29 yrs ago I too had a float problem (Marvel carb 0-200A) which my A&P fixed right away.

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