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Thread: Weird carb heat / rough engine

  1. #1
    Alex Clark's Avatar
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    Weird carb heat / rough engine

    Maybe it was just the air moisture but this was weird. This is the second time in the last couple weeks.

    The engine runs smooth at low RPMs and during full power climb out.... BUT during cruise rpm from 2100 to 2400 it runs a little rough like it is getting ice.

    It smooths out with about half carb heat....
    But it gets rough again immediately when carb heat is turned off. It does seem to increase over time like real icing.

    We tried leaning and it does seem to make any difference.

    It starts right up and idles a little lower than it used to.... And it seems to hesitate a little when advancing from idle to full power for take-off.


    Any ideas?

    Like maybe the mix is too lean and the carb heat makes it run rich? Maybe one of the clients yanked the idle shut-off too hard and he made the mix cable slip?

    I will see how it runs during the morning flight.... Maybe is was just the perfect conditions for ice...
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  2. #2
    cubdriver2's Avatar
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    "Like maybe the mix is too lean and the carb heat makes it run rich?"

    Sounds like a good place to start

    Glenn
    Last edited by cubdriver2; 06-24-2013 at 11:27 AM.

  3. #3

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    Induction leak? That's where I'd start. Mixture limits should be easy to check.

  4. #4

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    Mine did the same thing, I found the mixture control had slipped, it was just a very tiny bit from reaching the full rich stop. It was surprising, at least for me, that such a small adjustment corrected the problem.
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  5. #5
    Darrel Starr's Avatar
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    May be a replay of the problem Doctor Randy had:
    http://www.supercub.org/forum/showth...-heat-problems
    Carburetor screws loose allowing air to enter the carb.

  6. #6
    Alex Clark's Avatar
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    I just checked the carb bolts and they are tight. I also just cleaned the air filter again since we have a huge mosquito problem this year...

    I will check the mix cable

  7. #7
    aktango58's Avatar
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    fuel ingredient change?
    I don't know where you've been me lad, but I see you won first Prize!

  8. #8
    Alex Clark's Avatar
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    Not knowingly....

  9. #9

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    carb ice get it all the time ,does egt climb? that's a shure sign of ice pull carb heat watch cht fall

  10. #10
    Alex Clark's Avatar
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    No cht, no egt, no starter, no lights, no nothing.... Just oil temp and pressure, airspeed, Alt and a windshield.

  11. #11

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    Quote Originally Posted by Alex Clark View Post
    Maybe it was just the air moisture but this was weird. This is the second time in the last couple weeks.

    The engine runs smooth at low RPMs and during full power climb out.... BUT during cruise rpm from 2100 to 2400 it runs a little rough like it is getting ice.

    It smooths out with about half carb heat....
    But it gets rough again immediately when carb heat is turned off. It does seem to increase over time like real icing.

    We tried leaning and it does seem to make any difference.

    It starts right up and idles a little lower than it used to.... And it seems to hesitate a little when advancing from idle to full power for take-off.


    Any ideas?

    Like maybe the mix is too lean and the carb heat makes it run rich? Maybe one of the clients yanked the idle shut-off too hard and he made the mix cable slip?

    I will see how it runs during the morning flight.... Maybe is was just the perfect conditions for ice...
    I have delt with the same symptoms with a C85 and Stromberg carb mixture wired full rich. Past two weeks in the mountains..lots of moisture... Descend into the valleys and air is dryer and symptoms go away.
    Last edited by hooligan; 06-25-2013 at 06:10 PM.

  12. #12
    Dave Calkins's Avatar
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    Quote Originally Posted by Alex Clark View Post
    no nothing.... Just oil temp and pressure, airspeed, Alt and a windshield.

    ....and a windshield.

    Priceless!!!

  13. #13
    Alex Clark's Avatar
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    From the Alex Clark dictionary of aviation terms:

    CUB WINDSHIELD
    : see Cub Windscreen; (a) A device which allows an aviator to monitor the pitch attitude, bank angle and altitude of a Piper Cub type aircraft while in flight. (b) The device also warns the pilot of other aircraft within the flight area and provides constant weather service updates. (c) Some versions provide a secondary function as a cockpit interior illumination device.

    By the by: I tossed a couple shots of marvel oil into the tank, shook the wing and let it set for a few hours. Then we drained the tanks again and water some water and yuck come out of the tank sump. Marvel oil in the tank seems to attract all the yucky stuff together so it separates from the fuel. We had drained the tank sump many times before and found nothing. It was av-gas... Now she runs like a champ again...
    Last edited by Alex Clark; 06-26-2013 at 12:08 PM.

  14. #14

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    Now she runs like a champ again... [/QUOTE]

    I wonder if you put some of that in a Champ fuel tank if it would run like a Cub?

  15. #15
    aktango58's Avatar
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    Quote Originally Posted by sburg58 View Post
    Now she runs like a champ again...
    I wonder if you put some of that in a Champ fuel tank if it would run like a Cub? [/QUOTE]

    I was thinking that very thought!
    I don't know where you've been me lad, but I see you won first Prize!

  16. #16

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    drain carb bowl

  17. #17
    cubdriver2's Avatar
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    Quote Originally Posted by RAC cubs View Post
    drain carb bowl
    Ditto. And it was a 90hp Champ. He said it ran like crap when he tried to climbout after taking off. We were at a flyin and borrowed some tools and drained the carb and captured what we took out and it had about an ounce of water in it. Fixed it and it ran great but still didn't fly like a Cub

    Glenn

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