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Engine hesitates when opening throttles.

in 912 / 914 Technical Questions 11 years ago
My 912 UL in my Kitfox hesitates at about 3700 RPM when opening throttles at take off. Once it get past that point, it has good power.Any suggestions on what causes this would be appreciated.

  • Re: Engine hesitates when opening throttles.

    by » 11 years ago

    Hi Jerry,

    When is the last time you did a carb sync? When you do it use 3500 for the high end sync. It is far better than using 2500 that states in the manual.

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

      
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  • Re: Engine hesitates when opening throttles.

    by » 11 years ago

    I just rebuilt my carburetors and I tried to use a pair of vacuum gauges at idle to adjust the mixture. I had the cross over off and the ports blocked, but the engine shook so bad, one of the carbs over flowed. It runs fine with everything hooked up, except at about 3700 RPM. I,m not sure how to sync the carbs at that higher speed when the idle and full throttle are adjusted OK. Maybe you could enlighten me on this. Thanks.
      
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  • Re: Engine hesitates when opening throttles.

    by » 11 years ago

    Hi Jerry,

    Sounds like you synced the carbs at idle and full throttle. Many sync the higher rpm only at 2500 rpm and I was just trying to get you up away from 2500 to 3500. If you did it higher then no big deal. You don't really need to sync them at full throttle. If the engine was running that rough, but smoothed out after you hooked the cross over tube back up, it sounds like you had a leak when doing the sync. I'm kind of old school. I only take one cross over tube off on one side and plug my gauges in there. For me it's just easier and less fuss.

    It is possible you really don't have the carbs set up correctly at idle. Make sure each throttle arm goes all the way back. Make sure you have no intake air leaks during the sync. It is also possible you have a piece of debris in the idle jet. It's a very tiny hole.

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

      
    Report

  • Re: Engine hesitates when opening throttles.

    by » 11 years ago

    Jerry,

    You may be far enough out of sync at idle that the engine simply won't run well enough to do a pneumatic sync. You might try a mechanical sync at idle. How to directions are in the line maintenance manual. Once this is done, the engine should run a little rough at idle with the balance tube blocked but well enough to do pneumatic sync at 3500.

    There are different schools of thought on this, but I balance my carbs at idle. Once they are balanced I back the carb stop screw out until I can just see daylight between the throttle lever and the stop screw. Then I can fine tune idle speed with the throttle quadrant stop. Reason for this is that when I pull the throttle rapidly to idle, the quadrant throttle stop takes all the force and not the cables, carb throttle stops, or splitters.

    Reason for balancing at 3500 is that this is closer to where you will be running the engine most of the time and this is where you want the ideal balance.

    Reason for balancing at idle is that small differences in throttle plate position show larger differences in the gauge vacuum and can be more finely adjusted.

    You may have to bounce back and forth between idle stop screw and cable adjustments a time or two to get it just right.

    If you use the 3500 mark, you may want to adjust at idle first. This will get your engine running well with the balance tube blocked to go to 3500 and fine tune there.

    Bill.
    Thank you said by: Jerry Kohles
      
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  • Re: Engine hesitates when opening throttles.

    by » 11 years ago

    Thank you Roger and Bill. I will follow your suggestions and let you know how it turns out. It's going to be very windy here tomorrow so I will need to wait until Sunday to tie the plane down outside and make the adjustments
      
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