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Thread: Need help in diagnosing engine problem

  1. #1
    Join Date
    Jan 2017
    Location
    Fairhope
    Posts
    14

    Default Need help in diagnosing engine problem

    I need your help in diagnosing a problem with my Tri-Pacer which has an 0-290-D2 engine. Upon returning from a x-country yesterday and entering the landing pattern at my home airport, I pulled on the carb heat on downwind. The RPMs dropped dramatically and the engine started running very rough. I though it was carb ice and gave it time to clear. When it didn't, I turned off CH and the engine returned to normal.

    There had been no hint of engine trouble on the 2 hour trip home or in the runup including CH check at the departure airport.

    Once on the ground at home, I ran the engine up to 2200 RPMs and pulled on the CH. I left it on long enough to clear any ice. Again the same problem: extremely rough with CH on, smooth with it off.

    I let the airplane sit overnight and did a ground check this morning. I did a mag check at 1800 RPM which was normal. When I pulled on the CH, the RPMs dropped to 800 and the engine ran scary rough and was surging. Once again, when I shut off CH, the engine resumed smooth operation. Same when I switched fuel tanks.

    What could this be? Thanks for any help you can give me!

    William Bruce

  2. #2
    Join Date
    Oct 2013
    Location
    Lebanon, Mo.
    Posts
    189

    Default

    Will, I would have a A&P check the carb heat flapper in the carb air box. There is an AD there. Have it checked before flight and fixed before the next flight whatever the problem is.

  3. #3
    Join Date
    Nov 2006
    Location
    Mason, Ohio
    Posts
    350

    Default

    William,

    I'm not sure what AD Fred is referring to, but I could not find one that addresses the carb heat box. While it could be the flapper in the air box, My guess is that you have an exhaust leak inside the carb heat shroud on the right side of the engine. The manifolds are notorious for cracking inside the can that is welded to the manifold where you can't see them. The only way to really tell if it's cracked is to pull off the manifold and pressure test it. You could also have a leak where the manifold attaches to the muffler, although I don't think that would show up suddenly. A quick easy check is to pull off the flex tubing that runs from the carb heat shroud down to the air box on the carb. If the inside is sooty black or heavy gray, there is a leak in that system someplace. If there is a leak, the engine is trying to burn exhaust gas when the carb heat is pulled on and it doesn't run well on that stuff.

    Tom Anderson

  4. #4
    Join Date
    Jan 2017
    Location
    Fairhope
    Posts
    14

    Default

    Quote Originally Posted by Tom Anderson View Post
    William,

    I'm not sure what AD Fred is referring to, but I could not find one that addresses the carb heat box. While it could be the flapper in the air box, My guess is that you have an exhaust leak inside the carb heat shroud on the right side of the engine. The manifolds are notorious for cracking inside the can that is welded to the manifold where you can't see them. The only way to really tell if it's cracked is to pull off the manifold and pressure test it. You could also have a leak where the manifold attaches to the muffler, although I don't think that would show up suddenly. A quick easy check is to pull off the flex tubing that runs from the carb heat shroud down to the air box on the carb. If the inside is sooty black or heavy gray, there is a leak in that system someplace. If there is a leak, the engine is trying to burn exhaust gas when the carb heat is pulled on and it doesn't run well on that stuff.

    Tom Anderson

    Thanks Fred & Tom. The airplane is in for an annual and I've passed along your advice to the mechanic.

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