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Thread: Got me an attention grabber Carburetor MA-4SPA -32

  1. #1
    bvmbandit's Avatar
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    Default Got me an attention grabber Carburetor MA-4SPA -32

    All,

    What is causing the MS carbs to cause RPM loss at settings between 1200-1500 rpm? I've got a 57 Tri-pacer that no matter in the ground or in the air, I'm having a noticeable RPM loss every time it passes through 1400 rpm to the point that is growing progressively worse over the past 10 hours flight time. It seems to be starving for fuel at 1400 rpm otherwise runs great throughout the rest of the entire rpm range.

    What are you guys experiencing and what have you done to fix the problem for those of you that have come across this?

    I do understand that the transition from idle circuit happens between 1200-1500 rpm and that I may be looking at clogged idle tube perhaps upon further research...

    MA-4SPA -32 carb is what I have...

    Any input appreciated.

    Thanks,
    Vern
    Last edited by bvmbandit; 11-07-2019 at 03:47 PM.

  2. #2
    Glen Geller's Avatar
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    Default Re: Got me an attention grabber Carburetor MA-4SPA -32

    I have seen more than one plane where the screws securing the float bowl to the carb body have become loose, and the bowl chatters and leaks.
    Next time you run the engine note the throttle knob position at 1400 rpm (measure with a ruler or mark with a sharpie.)
    Fuel selector to OFF to positively starve the engine to avoid accidental start before proceeding.
    Pull the lower cowl and take a good look at the carb assembly to be certain all the hardware is secure.
    Look at carb linkages below the engine while a helper slowly moves throttle, look for interference or binding etc.
    You can also contact Lycoming tech support and describe your situation, they often have good insight and are very helpful, they might suggest a good carb resource as well.

    GG

  3. #3
    Gilbert Pierce's Avatar
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    Default Re: Got me an attention grabber Carburetor MA-4SPA -32

    The carb comes off the idle jet and transitions to the main jet and Venturi nozzle at approximately 1200 rpm. Low float level setting will delay the venturi’s ability to suck fuel thru the main jet thus severely leaning out the engine momentarily.

  4. #4
    bvmbandit's Avatar
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    Default Re: Got me an attention grabber Carburetor MA-4SPA -32

    Quote Originally Posted by Gilbert Pierce View Post
    The carb comes off the idle jet and transitions to the main jet and Venturi nozzle at approximately 1200 rpm. Low float level setting will delay the venturi’s ability to suck fuel thru the main jet thus severely leaning out the engine momentarily.

    Gilbert

    That's what it sounds like its doing. Thanks for the tip. I'll check it out...!

    - Vern

  5. #5
    Gilbert Pierce's Avatar
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    Default Re: Got me an attention grabber Carburetor MA-4SPA -32

    If your float was starting to leak or absorb fuel it would lower the float level and cause the symptoms you have.

  6. #6
    JPerkins's Avatar
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    Default Re: Got me an attention grabber Carburetor MA-4SPA -32

    Quote Originally Posted by Gilbert Pierce View Post
    If your float was starting to leak or absorb fuel it would lower the float level and cause the symptoms you have.
    Wouldn't the float sinking in the bowl open the needle valve and cause an over rich condition, not over lean?

    Sent from my SM-G930V using ShortWingPipers.Org mobile app
    Jason Perkins

  7. #7
    Gilbert Pierce's Avatar
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    Default Re: Got me an attention grabber Carburetor MA-4SPA -32

    After giving it more thought I believe you are right. If the float got heavier from absorbed fuel it would “float” at a lower level but the distance from from the float lever arm to the needle valve would increase and fuel level in the float bowl would increase.


    https://msacarbs.com/pdf/SB-2_B_RPLm...ats_062209.pdf
    Last edited by Gilbert Pierce; 11-09-2019 at 09:39 AM.

  8. #8
    Pacerfgoe's Avatar
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    Default Re: Got me an attention grabber Carburetor MA-4SPA -32

    Still, if the float is adjusted so the fuel level is low, your symptoms could show up.
    Forrest Miller

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