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It has been a horrible last 3 weeks.  I was taking my highlander with a rotax 912uls on vacation.  Got half way and stopped to top off fuel.  Their pump was broke so took off to the next airport.  Upon takeoff about 500 ft, the engine suddenly went from 5200rpms to 4800rpms.  I returned to the airport.  Did everything I knew to check.  Attempted again and same thing.  Called everyone I knew including Lockwood in FL.  Tried everything that I could do there.  No luck.  Had friends come and help.  Still no luck.  Drove the 4 hours home leaving my plane.  Everyone said that it had to be something in the carbs.  580 total hours with a carb rebuild done just last year.  So, i bit the BIG expensive bullet and bought two new carbs.  Drove the 4 hours back with friends and installed the new carbs.  Seemed to work on test flights!  Well everyone left and the next morning I attempted to fly home.  Same issue!  So decided to limp home.  Made it.  Have tried to drain the tanks with fresh gas, checked plugs, changed out mags, new carbs, no vapor lock, checked prop, checked exhaust, etc.  When it happens the fuel pressure is the same.  5+pounds.  The egt's drop a bit due to the drop in rpms.  Battery good.  Oh, and on the way home my volt meter went from 12+ to 0 in an instance.  Maybe not related.  I have had to walk away for a few days due to the frustration level.  I have thrown a lot of money at it with no results.  Everyone I speak with does not have a clue on how to help.  I am reaching out here in desperation.  

 

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  • Re: sudden drop in rpm's

    by » 9 months ago


    Now that you know it’s not the carbs. I’d start looking at the ignition system.


  • Re: sudden drop in rpm's

    by » 9 months ago


    If the mag drop check is okay then I'm still on the fuel delivery side. I don't think you needed new carbs. I wouldn't have thought the carbs unless they were really old, not clean and or haven't been synced for a long time. Carbs should be synced at every annual. 

    I had a Highlander across the hangar from me had the same issue. The A&P's chased electrical around for 2-3 weeks. I told them it was fuel. All I did was blow and flush out the reservoir tank and the rest of the fuel lines with some air from where the fuel line came off the tanks to the carbs. Never had another issue. My guys problem was in the reservoir. 


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


  • Re: sudden drop in rpm's

    by » 9 months ago


    To elaborate on Roger's thoughts...

    When the engine loses power, does it run rough any at all, or just smoothly operate at a lower rpm?
    The A and B mags will mask each other, and it is unlikely that both went slightly bad at the same time.
    A Mag check would reveal this problem.
    If either carb has a problem, the engine would run asymmetrical and rough.
    Again, it is highly unlikely that BOTH Carbs have the same problem.

    This leaves us with a fuel supply question.
    If the fuel supply rate is limited, the initially full carbs would slowly be depleted until the delivery matched the supply rate.
    This would produce full power for a short time after throttling up and then a reduced power once the extra fuel was depleted.
    This would allow you to limp home at a lower rpm.

    I suspect that after the engine slowed down, and you reduced power for the landing you would find that you once again had full power for a short time.

    Do you have an AUX Fuel pump?  A Fuel Pressure Gauge? 
    Have you checked your Fuel Filters?
    Check your fuel return line flow.
    Does the return flow drop to zero at full power indicating the Fuel Pressure at the Carbs has also dropped to zero?  (Pressure!, Not Flow!)


    Bill Hertzel
    Rotax 912is
    North Ridgeville, OH, USA
    Bill.Hertzel@Yahoo.com
    Clicking the "Thank You" is Always Appreciated.


  • Re: sudden drop in rpm's

    by » 9 months ago


    Fuel tank vent lines open and free of debris?


  • Re: sudden drop in rpm's

    by » 9 months ago


    Can you give me a quick how to on that?  Pull the fuel line off of both ends and run air through the lines?  

    Thanks,

    Dennis


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