Holding pilot air screw in place.

Discussion in '2-Stroke Aircaft Engines' started by keith103, Oct 19, 2019.

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  1. Oct 19, 2019 #1

    keith103

    keith103

    keith103

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    I seek your advise on pilot air screw. The pilot air screw on my Mikuni carb ( for engine Kawasaki 340 ) is wandering away from the desired position. I usually try to set it at 1.5 turns out, from fully shut position. But due to engine vibrations it keeps getting fully shut after a few minutes of operation. Is there some way to set at one place and prevent it from loitering around ? Am I missing something that is too obvious ? Thanks for your suggestions.
     
  2. Oct 19, 2019 #2

    TiPi

    TiPi

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    most adjustment screws on carbys have a relatively strong spring with sharp edges on the ends that prevents the screw from moving, something like a spring washer on a spring.
     
  3. Oct 19, 2019 #3

    pictsidhe

    pictsidhe

    pictsidhe

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    I'm rusty on Mikunis, but screws should be stiff enough to not vibrate around. O-rings or springs are a common way to do that.
     
  4. Oct 19, 2019 #4

    TiPi

    TiPi

    TiPi

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    yes, forgot about the O-ring. removed the screw and check that the O-ring (if fitted) is not hard and worn. Quite often they actually have both (O-ring and spring), might need them on a vibrating engine.
     
  5. Oct 19, 2019 #5

    proppastie

    proppastie

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    had to look at the YouTubes to see exactly which screw you were talking about.....Never had that problem so just winging it here...is the threads in the carb good, not a cracked carb body?...perhaps a new pilot screw would fit tight,... maybe a dab of non-hardening gasket sealer....form-a-gasket # 2 or 3 on the threads, do not get it on the needle or in the carb,.....last resort a removable thread sealer (blue lock-tite ?)....maybe the local motorcycle mechanic will have seen it before and have suggestions (besides a new carb)
     
  6. Oct 19, 2019 #6

    pictsidhe

    pictsidhe

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    No, do not use any goop. Fix it properly, or buy a new carb, which is what you'd need to do if you gooped it.
     
  7. Oct 19, 2019 #7

    keith103

    keith103

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    I appreciate all the responses. My plane (MiniMax) is at the airfield, so I dont have a picture of the carb right now. But the air screw is fairly snug, though not tight enough to prevent movement either way due to vibration.I am assuming there could be a small spring and gasket that fits into the screw hole, which keeps the screw from moving around. May be it just did n't come with my carb, or it could be an add-on accessory. My spark plugs were covered with a black deposit ( should be coffee color ) and I re-checked all other settings and jets, all of which looked OK, that is how I figured this jet was not staying in position. I will check with the engine vendor ( JBird ) to see if he can send me a screw with the spring and gasket. Thanks once again.
     
  8. Oct 19, 2019 #8

    proppastie

    proppastie

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    Or wrap a little bit of thread or masking tape around the threads.....sort of like a toothpick in a screw hole.
     
  9. Oct 20, 2019 #9

    pictsidhe

    pictsidhe

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    Now I think about it, I have a dim memory of stretching an adjustment screw spring that was no longer doing it's job.
     
  10. Oct 20, 2019 #10

    MadProfessor8138

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    I have seen the screw move over time even with the appropriate accessories....washer/spring.
    The washer & spring are suppose to prevent this from happening but they are working against quite a bit of vibration and often lose the battle no matter how new or well fitting the parts are.
    I wouldn't coat the threads with anything but you may try setting the screw in the proper position and then use RTV sealer or even clear silicone on the outside of the screw to hold it in position.
    Just a thought.....

    Kevin
     
  11. Oct 20, 2019 #11

    keith103

    keith103

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    An associated question is what purpose this screw serves ? Tightening / closing it enriches the mixture and opening it leans out the mixture, but is this only at low rpm’s like the pilot fuel jet does for fuel delivery ? Asking because the engine has done 13 hours without major issues till I noticed the screw is not seated right.
     
  12. Oct 20, 2019 #12

    delta

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    If it's the idle air screw we're talking about, it should be adjusted with the engine warmed up and idling. If the engine doesn't smooth out by adjusting it in or out, there's something else going on to give you the black plugs. Turn the engine off after you find the sweet spot . Note the direction of the slot, count the number of turns clockwise until seated. After you back it out exactly to where it was, find a way to keep it there. You could even safety wire it somehow if stretching the spring doesn't work.
     
  13. Oct 20, 2019 #13

    Hephaestus

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    Purple loctite? :) purely out of tbe box thinking and all...
     
  14. Oct 20, 2019 #14

    proppastie

    proppastie

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    adds air to lean at idle.....other possible issues for black plugs...needle in slide wrong height, gasket/head/base leak....black at bottom and white at tip of plug, stuck float, clogged air cleaner, choke or starter rich plunger not right, broken reed valve, if rotatory valve, bad o-ring on case,.... bad crank seal......air leaks often are unknown and compensated with higher mixture and slide settings to allow the engine to idle, this results in black plugs but when running and sucking more air will give the tip of the plug to be white, hence black at bottom white at the tip.
     
  15. Oct 21, 2019 #15

    keith103

    keith103

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    I was thinking I will just try some plumber's thread sealant tape. Or wrap a couple twists of just plain sewing thread. (Not too much that it will shred and go into the carb.) Also I think the pilot air screw affects the mixture only at idling and low rpm's. ( as Proppastie clarified.)

    I also figured out the plug fowling issue. Earlier I had checked only one plug. Later on, I decided to change both the plugs, and discovered that the other plug was perfect coffee color. So the loss of power issues, was not due to carb, but due to one defective plug which was firing only intermittently. With both plugs replaced, engine has good power and responds as it should. ( The plugs had only 7 hours on them. This is the plug that I had used: : https://www.amazon.com/gp/product/B01DMFHVJW/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1 )
     
    Last edited: Oct 21, 2019
  16. Oct 21, 2019 #16

    MadProfessor8138

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    Never use thread tape on a fuel system.
    This is common knowledge and should be adhered to.
    No matter how careful you think you've been with the thread tape....some of it will find its way into the carb.

    Kevin
     
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  17. Oct 22, 2019 #17

    proppastie

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    TFE thread tape is not very sturdy if not captured in the threads....and yes can get caught in the small passage ways.
     
  18. Oct 22, 2019 #18

    proppastie

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    good luck but there could be something wrong with one side of the engine, I say this because the mixture is good on the other cylinder.....but then again it is easy to foul a plug and sometimes the the simple solution is the right one.
     
  19. Oct 22, 2019 #19

    MadProfessor8138

    MadProfessor8138

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    With only one plug fouled I would look for the following......
    1. Check the plug to make sure it isn't breaking down under load....could be a bad plug ....years ago,NGK had a bad run of plugs and it drove myself and a few other guys crazy trying to figure out why we kept burning plugs after 2-3 hours run time.
    2. Check the plug wire to make sure it isnt breaking down under load....could be a bad plug wire.
    3. Check the plug cap to make sure it isnt losing contact with the plug....plug caps are notorious for this due to them wearing a bit from the vibration,they seem to snap on but they are actually a loose fit and will vibrate & lose contact while the engine is running.
    I zip tie or safety wire my plug wires to make sure they dont come loose.
    4. Check the coil.....make sure the plug wire is making good contact in the port.

    You posted that the engine is running properly now that the plugs have been replaced.
    You also stated that the bad plug was firing intermitantly......did you test this plug to find out if it was the plug itself or something in the ignition system causing the mis-fire ?
    Evidently you found the offending problem for the engine running rough......but I'm curious as to what caused the issue to begin with.....plug,wire,cap,coil...?

    Kevin
     
    Last edited: Oct 22, 2019
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  20. Oct 22, 2019 #20

    poormansairforce

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    I had a 340 on my Minimax also. Your problem is likely fuel distribution. Air can turn corners easier than fuel droplets so I had to twist the carb in the boot to ensure each cylinder got the same amount of fuel judged by EGT and plug color. Sounds like you may be rich if thats your problem. Wire the screw where you need it.
     

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