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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 » 12 years ago

    Engine mount rubbers are 1 year old. They do appear soft when rotating prop, I see the engine rotate on the compression peak and then recoil. Don't know if that's normal or not. I didn't replace the needle valve when I put the carbs back together. Went to the airfield and flew today and prior to that took off the float bowls and the right level was higher and when I removed the bowl there was a gush of fuel onto my hand. Tried bending the tab a small amount to set the level around the 12mm mark and flew, no change. Did not remove a plug to chk colour afterwards. I will look at a new needle valve first. Will also get some new engine mount rubbers ordered. Thinking along the lines that Roger mentioned, rough engine causing the carb to flood vs. carb flooding causing engine to be rough. I have exhausted almost all the options though and will probably end up having to live with it. Will look at a return line too. Last thought - could it be something to do with the gear box? It's the 2.43:1, no slipper clutch on the 80hp motor (yep, am sure of the ratio, bit unusual for the 80hp but that's what the importer for australasia brings in).

    Thanks.
      
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  • Re: 912UL Engine vibration on throttle back

    by » 12 years ago

    Hi Paul,

    Could be the gearbox, but not probable in your case. First replace the needle valves. Did you check the float level arms while the carbs were off? The carbs must be upside down to do this and the float arm should be 10.5mm from the edge of the bowl to the top of the float arm. This could be your flooding problem and or coupled with old or worn out float needles. When you rebuilt the carbs did you watch the Rotax video here on the forum?

    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 » 12 years ago

    yep, watched the video as have paid up the subscription. the video on disassembly/assembly is great. i didn't pull apart the starter circuit or remove the butterfly but did remove all jets and check. also squirted carb cleaner through the channels and used compressed air afterwards to check patency of the channels. only visually checked the float arms but will get new needle valves and re-check. still thinking along the lines of some harmonic shake as the vibe is so easily reproducible and comes clean at pretty much the same rpm each time (4200-4300). think maybe this ongoing vibe problem has now given rise to other problems - worn through throttle spring, broken exhaust springs x2, worn needle valve right carb, wonder what next...

    paul
      
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  • Re: 912UL Engine vibration on throttle back

    by » 12 years ago

    If one carb fuel level was higher than another then check the float arm height on both. You can use a mm ruler and set it on the edge of the carb bowl rim and slide it up to the float arm to check the height.

    Many people look for complicated answers when they are usually very simple. You just have to start at "A" then "B" then "C" and work your way through the issue. Most problems are simple, but if you bounce around or skip small details then you tend to go right past the problem.

    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 » 12 years ago

    I may have solved my vibration issue. I tried new float needle valves, no difference. Fitted brand name (barry) mount rubbers yesterday. The originals (no stamped numbers or ID data) were shagged out. Not only had the metal sleeve compressed/mushroomed but the holes were oval not round! These mounts came with the kit and the Skyranger forum mentions them as generics with aluminium sleeve which was too soft. I never managed to reach the torque setting on those old mounts as the rubber was getting too squashed appearing. The new mounts torqued up fine. Straight away on the ground I noticed a marked improvement with regards to vibration. In the air today and almost all the vibe is gone. I can cruise in an rpm range that before felt uncomfortably rough. Still get some roughness on final between idle and 3000 rpm but think I can live with that. I have not fitted a return line to the fuel line as yet but maybe that might help if fuel pressure getting a bit high on finals and slightly overpowering the float needle. I can still tell that the old vibration rpm range is different/less smooth but hardly compared to before fitting the new mounts. From what I have heard, there is often some "unhappiness" in these motors in the 3900-4400 range anyway. It is so minor I can easily live with it. Above 4400 the engine runs so sweet compared to previously, especially now I have a 3 blade prop. I am still cautious in saying the problem is fixed but feeling optomistic.

    Thanks all for the suggestions.

    Paul
      
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