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Thread: Marvel Schebler MA3SPA / O200 -- engine stumble

  1. #1

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    Marvel Schebler MA3SPA / O200 -- engine stumble

    When throttling for takeoff I've been getting a stumble around 1700 RPM.

    When the throttle is pushed forward normally (smooth / slow) the engine 'stumbles' for a couple seconds and then revs up normally. The engine smooths out around 1800 RPM.

    If the throttle is pushed forward quickly I can almost stall the engine.

    I've heard of this behaviour on the Strombergs but not Marvel Scheblers.

    Is it possible the accelerator pump is injecting too much fuel? Any ideas?

    thanks

  2. #2

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    Marvel Schebler MA3SPA/0200---engine stumble

    Does it do it all the time,hot,cold?If it will do it on a static run up then try it again with the mixture leaned slightly to see if that makes a difference.If for some reason the carb is rich then adding more fuel via the accelerator pump will make it break-up.
    Will it run up to full static rpm?
    Carb heat on?
    Dirty air filter?

  3. #3

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    Re: Marvel Schebler MA3SPA/0200---engine stumble

    Quote Originally Posted by willyb
    Does it do it all the time,hot,cold?If it will do it on a static run up then try it again with the mixture leaned slightly to see if that makes a difference.If for some reason the carb is rich then adding more fuel via the accelerator pump will make it break-up.
    Will it run up to full static rpm?
    Carb heat on?
    Dirty air filter?

    It does it all the time (although sometimes not as bad as others). It seems to be more noticeable when the engine is hot, but I should confirm this with a controlled test.

    The engine will static up to about 2500 with a 7140 prop.

    I will try with carb heat on / different mixture settings to see what happens.

  4. #4

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    Accelerator pump?

    Sounds like a faulty/worn accelerator pump. Try this: Instead of priming when performing a cold start, pump the throttle two or three times, then crank it. If it starts, then the pump is probably ok, if not, suspect the pump.
    I never have to prime either our Cessna 182, Smith Kit Supercub, or the wife's Champ. The only thing I do is pump the throttle three times while cranking the engine over. They always start just fine. I believe I can assume the accelerator pumps are functioning just fine.

    Mike

  5. #5

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    The carburetor was brand new this past July.

    It now has about 60 hours. Faulty maybe, but probably not worn.

    I'll try your test (skip priming... use accelerator pump).

  6. #6

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    A 0-200 war story, back in my cessna 150 days, the engine would run ruff and back fire at around 1700 rpm. Ok, everybody say intake leak. Pressure test the intake system with air a soap bubbles and couldn't find anything. after swapping carburetors and other ways of spending money we started removing things till we found the problem, found it pretty quick. The problem was one of the gaskets on the elbow of the intake pipes had shrunken inward on one of the sides. When the air pressure was applied it force the gasket out and sealed things enough so soap bubbles would not form.

    Just a war story for what it is worth. Jim

  7. #7
    Steve Pierce's Avatar
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    Where did you get the carb. I have gotten great warranty service 2 plus years after the overhaul from Kelly.
    Steve Pierce

    "When everything seems to be going against you, remember that the airplane takes off against the wind, not with it."
    Henry Ford

  8. #8

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    It's most likely either the throttle pump leather or crud in the throttle pump outlet tubes. However, since it is easier and quicker to check for induction leaks, I'd do that first. Carb to spider, spider to each induction tube, and each induction tube to cylinder.
    JimC

  9. #9
    behindpropellers's Avatar
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    Re: Marvel Schebler MA3SPA / O200 -- engine stumble

    Quote Originally Posted by gcgilpin
    When throttling for takeoff I've been getting a stumble around 1700 RPM.

    When the throttle is pushed forward normally (smooth / slow) the engine 'stumbles' for a couple seconds and then revs up normally. The engine smooths out around 1800 RPM.

    If the throttle is pushed forward quickly I can almost stall the engine.

    I've heard of this behaviour on the Strombergs but not Marvel Scheblers.

    Is it possible the accelerator pump is injecting too much fuel? Any ideas?

    thanks
    Are you sure it is a Marvel carb?
    Piper J-5A C-90 N40877
    J-5 Project Pictures

  10. #10

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    Actually it's a Precision carb... model is MA3SPA.

    Same as Marvel isn't it?

  11. #11

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    It is the same. and the problem is pretty much typical on these engines. It can also be caused by a worn throttle plate support and holes.
    JimC

  12. #12

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    marvel

    take jim's advice, i had the same problem 10 years ago and that fixed it

  13. #13
    cruiser's Avatar
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    http://www.sacskyranch.com/eng195.htm I would be sure the carb has been serviced to comply with SB # MSA-7.

  14. #14
    Greg Smith's Avatar
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    Quote Originally Posted by Jim Mizer
    ...The problem was one of the gaskets on the elbow of the intake pipes had shrunken inward on one of the sides. When the air pressure was applied it force the gasket out and sealed things enough so soap bubbles would not form... Jim
    Next time try squirting a touch of carb cleaner on each joint with the engine running. If the RPM increases, you've found the leak.

  15. #15

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    I tried accelerating through the rough RPM range with carb heat applied.

    The problem seems to either go away or is less significant (I wasn't able to do much testing).

    I'm thinking maybe the cold weather has resulted in my mixture being too lean ... application of carb heat enriches the mixture .... problem seems to go away?

    After a little more testing to confirm my theory I plan to adjust the idle mixture (enrich). I'm not sure if this will work since the stumble seemed to be occuring during the transition from the idle circuit to the main jet (or slightly after).

    Although I haven't confirmed I feel the accelerator pump is working.

  16. #16
    JPachl's Avatar
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    Check your intake gaskets. Inspect your intake tubes for visible leaks. It doesn't take much. These malfunctions could cause your problem. They did on my engine.

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