Marvel Schebler Carbs

Discussion in 'Firewall Forward / Props / Fuel system' started by spduffee, Sep 13, 2015.

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  1. Sep 13, 2015 #1

    spduffee

    spduffee

    spduffee

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    Hello,
    I have been looking for an MA3 SPA for my Corvair. I noticed there are several model numbers - some used on Lycomings, others on Contis or Franklins. What is the underlying difference in the models? They are all MA3 SPA's. Is there any specific one I should be seeking? WW's manual simply says he prefers the MA3 SPA, but no specific model number.
    Help, please?
    Thank you,

    Shawn
     
  2. Sep 14, 2015 #2

    Daleandee

    Daleandee

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    Shawn,

    I have a Corvair on my Sonex. I run the recommended carb which is a Marvel Schebler MA3-SPA 10-4894. That number is superseded by 10-4894-1.

    There's several places to source one from. Here's an example:

    MARVEL SCHEBLER 10-4894 CARBURETOR ASSY MA-3-SPA ( 1 Piece Venturi SEE DETAILS )

    Of course then there is the whole one piece, two piece venturi thing to argue about ... :ponder:

    Dale Williams
    N319WF @ 6J2
    Myunn - "daughter of Cleanex"
    120 HP - 3.0 Corvair
    Tail Wheel - Center Stick
    Signature Finish 2200 Paint Job
    98.3 hours / Status - Flying
    KITPLANES Newsline
    https://www.youtube.com/channel/UC49h6Qijc17_Ebfz0CbRFtg/videos
     
  3. Sep 14, 2015 #3

    djschwartz

    djschwartz

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    The model numbers have to do with several things, among which is the main jet size. The only way you'll figure out which one might work on a Corvair is to find someone who has used one successfully on a conversion very similar to yours or to just buy one and start experimenting.

    Dave
     
  4. Sep 14, 2015 #4

    spduffee

    spduffee

    spduffee

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    Thanks for the replies. So, I am looking at a 10-2971, which is, I believe, from a Franklin. The engine had an MA3SPA on it before, so at least the manifold is geared for it. I just wonder if this particular carb will work for me....
     
  5. Sep 14, 2015 #5

    Avid90

    Avid90

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    The 10-2971 carburetor was for a 150 horsepower 6 cylinder Franklin engine of 335 cubic inches displacement. It's primary venturi part number was A46-A33, main venturi part number A46-A65, nozzle and power jet assy A47-668, and economizer jet A49-A65. The venturi part numbers are for the original two-piece venturi. Go to the Marvel Schebeler web site and look up bulletin MSA-2. There you will find the part numbers and some applications for the one piece venturis. You may find that your original carb is too much for the 164 cubic inch Corvair engine, and have to change out the venturi and nozzle to one that is more suited to your engine.
     
  6. Sep 14, 2015 #6

    spduffee

    spduffee

    spduffee

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    Ok, I understand that now. Thanks for the example and explanation.
     
  7. Sep 14, 2015 #7

    N8053H

    N8053H

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    Its my understanding from the posts on this forum about these carbs, That if you use the one piece venturi your engine will stop running or could. Not sure if I got that info correct though. I have one of these carbs on my a-65. I am really reluctant to fly this airplane again because of this carb. and what was posted on this forum about it.

    Some pics of my carb and the venturi.

    carb venturi 004.jpg
     
  8. Sep 14, 2015 #8

    mcrae0104

    mcrae0104

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    Shawn, do you have an older manual? I'd strongly recommend you get the 2014 one, which does list the specific model. I'll send you a PM.
     
  9. Sep 14, 2015 #9

    TFF

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    http://www.kellyaerospace.com/articles/Accessory_AMT.pdf When the AD came out, just changing the venturi caused problems on some engines. The fix was a new discharge tube that needed to match the new style venturi. Throwing parts at anything usually does not fix problems which is what Mar Sheb found out; they needed a whole package to make it work.
     
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  10. Sep 14, 2015 #10

    spduffee

    spduffee

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    Thanks all for your comments. I figured I had better just call MS to hear their take. Here is a summary of the technician's comments:

    As mentioned by Avid90 above some of the components may need to be changed out: venturi, nozzles, jets and idle tubes. For example - the 2971 has an economizer-jet, whereas the 4894 does not. Exchanging this may improve performance. The 2971 also has an accelerator pump ( SPA) He stated the throttle body lid may be different. In general, the technician stated that if it were his plane, he would use the 2971 as is and experiment with it, making adjustments and or exchanges, if needed.

    To N8053H - My old C150 had the two-piece replaced with the one-piece (per AD) before I bought it. I flew it for 50 hours before selling it. Flew like a champ.

    I am still not sure if a low-priced 2971 would be a good deal after all, but it may be worth the risk.
     
  11. Sep 14, 2015 #11

    Avid90

    Avid90

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    To H8053H: As I tried to state in your original post (in another thread), change the nozzle as per the bulletin, updating your carb to the -1 model, and your rough-running problem will possibly be fixed. But keep in mind that your carb is sized for an 0-200. It may not work well with your A-65. You may have to change the venturi as well as the nozzle. See the Marvel Schebeler bulletin MSA-2 for what you might need.
     
  12. Sep 15, 2015 #12

    mcrae0104

    mcrae0104

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    Why take a risk? Copy other people who have been successful. Use the one in the manual. Your life is worth the cost difference (if there even is one).
     
  13. Sep 15, 2015 #13

    Daleandee

    Daleandee

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    I wholeheartedly agree! When I built my "Cleanex" I followed Dan Weseman's and Chris Smith's examples very closely as they had very successfully paved the way forward. What are you putting your Corvair on?

    "Buy the best and cry once!"

    Dale Williams
    N319WF @ 6J2
    Myunn - "daughter of Cleanex"
    120 HP - 3.0 Corvair
    Tail Wheel - Center Stick
    Signature Finish 2200 Paint Job
    98.3 hours / Status - Flying
    KITPLANES Newsline
    https://www.youtube.com/channel/UC49h6Qijc17_Ebfz0CbRFtg/videos
     
  14. Sep 15, 2015 #14

    spduffee

    spduffee

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    Of course you're right. I will keep looking for the proper unit.
    What is the Corvair going on? A Flybaby, which may be an original or a tandem - not decided yet. Doesn't matter right now, as I, we, are working on a far more important project (one that only takes 9 months to build but for which I will pay a lifetime). I promised the real builder of this project that I would repress any airplane nonsense for a while. Doesn't mean I can't work on some old useless engine that I have had in the garage for almost two years...
     
  15. Sep 15, 2015 #15

    Dan Thomas

    Dan Thomas

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    Here's the Airworthiness Directive that addressed the two-piece venturis. It explains the problem:

    Precision Airmotive Corporation Carburetors

    The original directive demanded that the two-piece venturi be replaced by a one-piece, since the inner venturi of the tow-piece could come loose and get sucked into the engine. It was held in place by its three little legs being pinched between the upper and lower carb sections, and as the gasket compressed it could work loose and gradually wear away the tips of those legs. Once it fell out, the fuel nozzle quit delivering fuel.

    But a problem with the one-piece arose. Some engines ran roughly with it, and one rebuilder showed me that the legs of the new venturi had squared-off faces that turbulated the incoming air. Sort of like having flat leading edges on your wings and sharp corners long the top and bottom. Dumb. Anyway, a second AD demanded that a new fuel nozzle, with more holes in it, be installed to fix the rougnness. It made things worse in some cases and better in others. So this final AD came out to allow you to reinstall the two-piece affair and inspect it annually to make sure it wasn't loose. I tried to find those parts to fix the carbs on the rough-running O-200s we had, but they told me that they'd all been destroyed. Hmph.

    Some rebuilders would smooth and polish those squared-off legs and solve the problem.

    Go here and scroll down to the top of page four:

    http://www.kellyaerospace.com/articles/Accessory_AMT.pdf
     
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