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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

    Dear Forumites

    The springs under the diaphragm are supposed to be about 119mm long. Mine where 108mm. I stretched them back to their original length and after that vibration problem never returned. The carb overhaul kits were just installed as a precaution. The EGTs are now well within range and very equal across all cylinders. All spark plugs are now white and I will still try and adjust the main needle accordingly. The Rotax heavy maintenance manual prescribes the clip in slot three on a 912 UL, the local Rotax guys say leave it in slot 2. Mine is still in slot 2. But this would anyway just address any mixture issues in the part-load range. The main jets are 158.

    Mike, the wrong rotary valve on the one carburettor definitely admitted some false air into the system but I think the fuel ports on the rotary valve remained closed. Be that as it may, the vibration had disappeared before I discovered the rotary valve mix-up, so that wasn't the man cause of the problem. The engine starts immediately.

    Roger, I will replace the wrong rotary valve, but right now I'll wait until my bank balance has improved somewhat.

    Blue skies
    Arthur
      
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  • Re: 912UL Engine vibration on throttle back

    by » 11 years ago

    That's great info on the large springs, Arthur. Thank you! I'll be checking mine soon. I've learned a lot from this thread about just how much engine vibration is related to the carbs and fueling. Mike
      
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  • Re: 912UL Engine vibration on throttle back

    by » 11 years ago

    Hi Mike

    I wouldn't advise trying to stretch your carb slide springs. They should be the same length as new ones. If they are shorter, then you need to replace them with new. The slide is pulled up by a vacuum and is very delicate. If you stretch the springs you will alter the pre tension on the slide and this will alter your mixture.

    Stretching the springs as Arthur has done wouldn't be the reason the vibration has gone, it would be to do with worn parts and split o-rings being replaced.

    If your engine is running OK, leave it alone.

    Mark
      
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  • Re: 912UL Engine vibration on throttle back

    by » 11 years ago

    Hi Mike

    I wouldn't advise trying to stretch your carb slide springs. They should be the same length as new ones. If they are shorter, then you need to replace them with new. The slide is pulled up by a vacuum and is very delicate. If you stretch the springs you will alter the pre tension on the slide and this will alter your mixture.

    Stretching the springs as Arthur has done wouldn't be the reason the vibration has gone    , it would be to do with worn parts and split o-rings being replaced.

    If your engine is running OK, leave it alone.

    Mark


    I'm not sure why this comment is addressed to me. However, I think you missed the point of Arthur's comment which was that equalizing the length of the slide springs by stretching one of them WAS the cure for his engine vibration. If the springs are exerting unequal forces on the slides then the mixture will change differently in each carb at different power settings and at different altitudes.

    Since Arthur reminded me I remembered I have in my files another example of the exact same situation with the exact same fix. This was on a brand new engine, fwiw.

    I'm sure there's a spec. for the nominal spring length floating around somewhere. I have the Bing 64 manual - not to hand unfortunately - and will check it to see what the official word is.

    I guess I'll add one more thought. Whether you think spring stretching is a good idea or not, there's no reason not to measure the lengths of your slide springs. If they're the same, great, if not, it's useful information.
      
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  • Re: 912UL Engine vibration on throttle back

    by » 11 years ago

    :whistle:
      
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