Schebler MA-4 Carb / O-320

Discussion in 'Maintenance Bay' started by Skymac, Jul 19, 2020.

  1. Skymac

    Skymac Pre-takeoff checklist

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    I’ve got a 172 that just doesn’t idle well below about 1200 rpm, I believe it’s running pretty rich. It’s not unsafe, I just know a O-320 with idle smoother. I know the throttle shaft has some play, do these usually tighten up with replacing just the bushings, or does the throttle shaft need to be replaced with the bushings? The idle mixture screw seems to back out after it runs for a few hours so I’ve ordered a new retaining spring for that as well.

    Thoughts?? Also, I haven’t been able to find a MA-4 parts diagram online, does one exist?
     
  2. GRG55

    GRG55 Final Approach

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    Sounds like you have an idle mixture issue, which you've already identified. I would be inclined to fix that first, get the idle mixture set for your conditions and then see if you need to do anything else to the carb.
     
  3. kyleb

    kyleb Final Approach

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    If it is idling rich, there is an idle circuit adjustment screw on the carb - a knurled screw that is on the aft side of the carb, towards the top. Turn it in (Right) to lean the idle mixture, turn it out to enrich the mixture. Best place to start is to screw it all the way in, counting how many turns it takes. If you think it is idling rich and you know it was adjusted 3 turns out, you might set it a 2 turns out and check results. It is an iterative process.

    Also, the idle circuit controls how the engine shuts down. At a nominal 1000 rpm, if you do a slow pull of the mixture lever, you want to see an RPM increase before the engine dies. A 20-50 rpm increase is a reasonable result.

    So you're in in iterative process to find optimal - where the engine idles well AND has an RPM rise before shutdown. You may tweak that idle adjustment screw in or out a half dozen times to find the ideal setting.

    Alternately, your problems could be caused by wear in the carb (not just the throttle shaft) or by induction leaks. That's a path to chase once you've tried adjusting the idle enrichment screw.

    All of these things should be done with an A&P's help since this is a certified airplane.
     
  4. Skymac

    Skymac Pre-takeoff checklist

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    Yea I’m working with an A&P and I might as well be one minus the testing, at the end of the day it’s good to know if anyone is super familiar with the carb on the setup. I’ve rebuilt 100 carbs before but it’s always nice to be able to ask for input.
     
  5. Salty

    Salty Final Approach

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  6. Tom-D

    Tom-D Taxi to Parking

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  7. Dan Thomas

    Dan Thomas Final Approach

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    People ALWAYS blame the fuel system first. It results in throwing expensive parts at a problem until someone finally suggests looking at the elecrical stuff. Poor idle is often due to the real culprit in 90% of performance problems: the ignition system. Weak or intermittent spark. Low RPM generates a weaker spark, and if those mags haven't been off for a long time they should be removed and inspected. Points erode, shifting the internal timing, and the spark weakens. Champion sparkplugs are famous for failures of the internal resistor, something they fixed a few years ago, but there are still a lot of old plugs flying around and probably some old stock still on vendor's shelves. Ignition leads can start shorting internally.

    Mags are supposed to be a 500-hour item (Slick) or 400 hours (Bendix). Sparkplug cleaning and pressure bomb testing is a 100-hour thing. Checking the leads is a 100-hour thing.


    An induction leak can also cause lousy idle.
     
    Last edited: Jul 19, 2020
  8. Tom-D

    Tom-D Taxi to Parking

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    ???
     
  9. Dan Thomas

    Dan Thomas Final Approach

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    He didn't say that the idle improved with idle mixture adjustment. He just said it won't idle well below 1200 RPM and that the idle mix screw tends to back out. The two are not necessarily related.
     
  10. kyleb

    kyleb Final Approach

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    Which makes me think induction leak.
     
  11. Tom-D

    Tom-D Taxi to Parking

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    I guess that is why he order a spring.
     
    Last edited: Jul 20, 2020
  12. Tom-D

    Tom-D Taxi to Parking

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    anyone know if he has the correct carb..

    Let's try guessing what is wrong without any clue :)
     
    Last edited: Jul 19, 2020
  13. Skymac

    Skymac Pre-takeoff checklist

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    Haha I love the Internet,

    Mags have about 75 hours since new, and it’s a “rich” issue, plugs could probably stand to be replaced though, they are getting to be high time. I’ll report back, I just thought it would be helpful if there was a good resource on carb info that I may have missed from experimental guys, etc.
     
  14. bluerooster

    bluerooster Pattern Altitude

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    Pull the carb and "IRAN" it. Then check for induction leaks while waiting on carb parts. If your mech has a spark plug tester, put it to use. But I kinda doubt plugs to be the trouble. I think it may be in the idle circuit in the carburettor. If you're thinking too rich, lean it and see if there's an improvement. This time of year, I lean aggressively on the ground. I found that it prevents fouled plugs.
     
  15. kyleb

    kyleb Final Approach

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    Shorty, other than the adjustment screw, is there anything that can be done to enrich the idle on one of these carbs?
     
  16. bluerooster

    bluerooster Pattern Altitude

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    Not that I know of. But it can be leaned via the mixture control knob. The OP mentioned that he thought it to be too rich.
     
  17. Daleandee

    Daleandee Cleared for Takeoff

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    I'm a fair mechanic but I believe carbs should be sent to a qualified rebuilder or order a rebuilt. These carbs can be considered to be quite simple to rebuild but it's the small things that can get you. I wouldn't rebuild my own mags or carb. It's worth the money to have a professional do it. If it was good and there is now a change it's because something changed.

    Here is a story of a man who fought with his Marvel Schebler carb ... and lost - https://flycorvair.net/2013/04/20/risk-management-judgement-error-money-in-the-wrong-place/
     
  18. Domenick

    Domenick Line Up and Wait

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  19. Tom-D

    Tom-D Taxi to Parking

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    Remember,, rebuild means it was tested IAW a approved method.

    who has a carb flow bench? (43.2)
     
  20. Domenick

    Domenick Line Up and Wait

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    Does it count that I still have a manometer I used for my 1967 Datsun 1600 sidedraft SU carbs?
     
  21. bluerooster

    bluerooster Pattern Altitude

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    And IRAN means "Inspect, and Repair As Needed". I've IRANed many carbs, and actually tested a few on the bench. I've never re built, or overhauled any. Sometimes IRAN includes replacement of everything except the casting.
     
  22. Tom-D

    Tom-D Taxi to Parking

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    Or repaired.
     
  23. RI Aviation

    RI Aviation Filing Flight Plan

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    I would also check the float for leaks & the condition of the needle valve.
     
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  24. Arnold

    Arnold Cleared for Takeoff

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    I resemble that remark.

    I've still got one from my 1973 Z car.
     
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