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Removal and reinstallation of throttle butterfly

in General Tech Discussion 6 years ago

  • Re: Removal and reinstallation of throttle butterfly

    by » 6 years ago

    You are correct. You must use the proper fixation screw that is used with the "O" ring ring.

    Roger Lee
    LSRM-A & Rotax Instructor & Rotax IRC
    Tucson, AZ Ryan Airfield (KRYN)
    520-349-7056 Cell

      
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  • Re: Removal and reinstallation of throttle butterfly

    by » 6 years ago

    Thank you Roger. On the Rotax carb assembly video they show the use of a metal bar, circular in diameter and with a slight chamfer drilled into one end which sits in a vice for the end threads of the butterfly fixation screws to be peened over against with the use of a drift. How deep should this chamfer be? Should it be just deep enough for the screw head to sit in to help prevent slippage, but still physically touching the screw head, or does it not matter whether it touches the head or not? I am concerned that if the bar does not touch the screw head then impact from the hammer and drift whilst peening the screw end thread might damage the internal threads of the butterfly shaft (or even bend it?). However, having seen on other carbs where the screw heads have been damaged. making it very difficult to get them out, I don't want to damage the screw head either as they seem incredibly soft. Rock and a hard place springs to mind! Small soft screws + brass shaft = VERY DELICATE JOB!
      
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  • Re: Removal and reinstallation of throttle butterfly

    by » 6 years ago

    When you put the butterfly into the slot in the shaft it has a center punch mark on the top of the butterfly. That should be facing up and outward. The shaft itself has recessed screw holes for the screw heads to tighten down into. Make sure when you tighten these screws that the butterfly is in a perfect position to seal all the way around and it isn't cocked off slightly. Otherwise the carb may not shut off completely. I use a 1 3/8" x 6' long aluminum bar stock that I clamp into the vise. I set the carb down over the rod so the screw heads and butterfly bar are resting and supported by the bar and can't get damaged. The 1 3/8" is a perfect fit for the carb throat. Then I take a long punch and peen over the screw threads poking out the other side to prevent the screws from ever coming out. The last thing you want is a screw coming out and bouncing around in the cylinder. You don't need pound the slotted side of the screw into oblivion that sits on the bar. The screws are soft. Just peen them on the threaded side. I have seen some screws I can screw right out, some I have to file down on the threaded side and some I had to heat quite hot to get them to turn. I always tell people don't use force to put things together and use as minimal force as necessary to disassemble. You don't want to use so much force you twist the slotted head out either putting them in or taking them out.

    Roger Lee
    LSRM-A & Rotax Instructor & Rotax IRC
    Tucson, AZ Ryan Airfield (KRYN)
    520-349-7056 Cell

      
    Report

  • Re: Removal and reinstallation of throttle butterfly

    by » 6 years ago

    Thanks Roger for your advice, it was most helpful. I put the first carb back together today and found that with a mix of a little jiggle here and a little tap there and some heating also (I am working in a freezing cold hangar) it all went ok. In the process I also found multiple reasons for the initial problem of leaking fuel so the job was well required. Just one question. Would you use loctite as a matter of course on the majority of threads or only where specified in the manual. Apart from the jet needle fixation screw and the four cross head screws for the choke housing loctite I believe is not specified for use anywhere else on the 912 carb and isn't shown used in the video tutorials. I don't like to over tighten threads but how do you know you have gone tight enough when there are no torque settings, no loctite and no locking aids (such as spring washes or nyloc nuts)? I guess it is just a bit of paranoia about things coming undone!
      
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  • Re: Removal and reinstallation of throttle butterfly

    by » 6 years ago

    Okay I know this may end up sounding kind of dumb, but...
    Tighten the screws until they are snug. If you have to apply lots of pressure and come close to stripping out a screw head that's way too tight. I have never ever had a screw come loose. The screws will bottom out and feel tight so stop right there.
    I have had quite a few carbs come in with the top dome phillips head screws so tight I almost couldn't get them out. I then use my small hand butane torch that I can aim the tip of the flame and heat the surrounding metal hot. I like that small torch because I can aim the tip of the flame right where I want it vs a heat gun that heats everything in a wider area.
    To each his own on this tip. Then most times if I tap the screw head with the screwdriver in it it will loosen and I can remove it. The tapping helps loosen any corrosion and the vibration helps loosen the over tightened threads. If I have to do something like this then someone really wrenched down hard fearing the screws would fall out or they would have a leak. The fixation screw down inside the piston (slide) that holds the needle in place has Loctite on it and I have seen many twist the head of the aluminum screw off. What a PITA it is to get out then. I always use that small butane torch to add some heat down around that screw to loosen it and then it just easily comes out.
    As far as Loctite usually many use too much and cover all the threads. "Just a dab will do You" as the old Brylcreem hair cream commercial used to say.

    Roger Lee
    LSRM-A & Rotax Instructor & Rotax IRC
    Tucson, AZ Ryan Airfield (KRYN)
    520-349-7056 Cell

    Thank you said by: Rob Seaton
      
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