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912UL Engine vibration on throttle back

in 912 / 914 Technical Questions 12 years ago

  • Re: 912UL Engine vibration on throttle back

    by » 11 years ago

    Hi Paul,

    Hope it works out for you.

    Engine mounts play a big part in vibration. Everyone should check the torque on the bolts from time to time because they can get loose. As mounts wear they may also shrink over time just enough to allow the mount to become loose. You may not even feel it. (Hand tight, but wrench loose scenario) Over time the holes that the bolts go through on the mount mat egg out some due to hard landings and you'll never see it. Mounts get harder over time just from age and again you'll never know and pushing on them is no way to tell. I hear all the time that the owner pushed on them and they seemed okay or not rock hard. Not a good way to test or check mounts or the hidden holes with the bolts through them.
    It is important to change them out every 5 years. This makes sure the rubber is the right hardness to dampen the vibration and help reduce engine movement. this allows you to look through the mounting holes to check for egging bolt movement.
    Just because a mount has rotted off yet doesn't mean it isn't bad.

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

      
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  • Re: 912UL Engine vibration on throttle back

    by » 11 years ago

    Hi Roger

    I am gonna fit a return line as still have some carb flooding issues on descent at idle despite new float valves. As mentioned before I have the new style pump. Is the restrictor hole size critical? I hear it is meant to be 0.35 mm which doesn't sound that big or big enough to make much difference. I have fabricated a restrictor by soldering over a brass T one end and drilling a hole, 0.4mm being the smallest I could use. Any idea how big a hole can be before the carbs get fuel starved from the return line?

    Paul
      
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  • Re: 912UL Engine vibration on throttle back

    by » 11 years ago

    Hi Paul,

    The return line is a good idea. I personally don't know if the increase to .4mm will make a big difference. I have never tried it. I can only guess, but with the new style pump I don't think so. You have my interest peaked. If you use it I would like to know the results. That orifice size was originally set up for the old pumps. Where are you going to run the return line back into?

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

      
    Report

  • Re: 912UL Engine vibration on throttle back

    by » 11 years ago

    I will first try returning fuel in between the shut off valve and filter. This is not far from the tank feed point behind the seats. The Skyranger tanks sit behind the seats and are joined at the bottom with a T and single feed. There would be slight gravity pressure at this point just from the head of fuel. The other place to return it would be via the screw on lid of the right tank but this would be a little more work and make any future plans to install a fuel flow meter tricky, not that I really need a flow meter. A friend has an 80hp storch and it has a return line to the left wing tank. If both tank valves are open and on the ground with engine running it quickly returns enough fuel to overflow the left tank breather if starting with near full tanks, makes me think that the return line on that plane has a much larger restrictor hole! Will head to the hangar today and put the return line in and let you know how it goes.

    Paul
      
    Report

  • Re: 912UL Engine vibration on throttle back

    by » 11 years ago

    Running a return line has now appeared to stop the descent under idle power flooding of the carbs. I still am vibration free in the 3900-4400 range after changing engine mount rubbers. As we go into winter here, the engine has been running pretty cool and still a bit rough at lower rpms so I taped off 1/4 of the water radiator and it runs much better, still only 83degC as the hottest CHT so could probably tape some more but will see how it goes. On the whole, after flying this morning, I am the happiest I have been with the motor. Touch wood.

    Paul
      
    Report
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