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I have a new 914 install on my Kitfox.  The engine has 9 hours on it.  After flying 2:25 and landing for fuel, I tried to take off and at around 4100 RPM the engine won’t accelerate.  Any additional throttle and the RPM decreases and it wants to die.  I noticed some fuel residue in the right hand fuel drip tray.  

  • Re: 914 won’t go past 4100 RPM

    by » one month ago


    This sounds like a science question for a non scientist.

    Engine power requires fuel in the correct fuel air ratio for combustion -  more fuel:air, more power.

    Assuming no internal engine problems - Rotax RPM is regulated by the available power, the pitch of the prop, climb angle, forward movement of the aircraft, etc

    I assume nothing has changed regarding the prop pitch . 

    Do the simple, usually cheaper stuff first - Check that the throttle cables are free and working as expected through their full movement. Throttle regulates air:fuel mixture, therefore power. If one or both carburettors are not going to full throttle, when you ask for it, you will get reduced RPM.

    Then check for air leaks & delivery - Air filters clean, Carburettor rubber flanges secure. Turbo pressure air hoses in good condition & secure, Air restriction or loss (turbo pressure) will reduce air availability, therefore power.

    Then check for fuel delivery problems- Fuel filters clean (used compressed air &/or reverse flow of cleaning solution, visual check alone not sufficient). All fuel lines in good condition & secure (no air leaks). No kinked hoses. Fuel pumps all working as expected. Float bowls clean & floats correct weight and floatation level. Float bowl gasket in good condition. Float bowl secure. Carburettor jets clean. Any reduction in fuel supply will reduce power & may also result in elevated EGT 

    Check ;- No restriction on inlet/exhaust -  Muffler has not had an internal collapse (shake & listen). Turbo wheel free moving without "play" & blades all in good condition. Ignition system secure. Leads all in good condition no loose connections or worn insulation - special attention to trigger coils.

    Good luck


  • Re: 914 won’t go past 4100 RPM

    by » one month ago


    Thanks for the reply.  I did most of that.  But upon reading more forum posts everything is pointing to over torqued float bowl nut.  Going to check out the bowl today.


  • Re: 914 won’t go past 4100 RPM

    by » one month ago


    Clyde Poser wrote:

    Thanks for the reply.  I did most of that.  But upon reading more forum posts everything is pointing to over torqued float bowl nut.  Going to check out the bowl today.

    Interesting - please let us know what you find.


  • Re: 914 won’t go past 4100 RPM

    by » 6 weeks ago


    The problem was finally found.  Through the system knowledge of a terrific Rotax source I was able to locate a crack in a 1" piece of rubber hose that went from the 3 way solenoid valve to a tee.  This hose is part of the carburetor enrichment that is programmed through the TCU.  There is a fitting on the airbox that take pressure and routes it through that valve and when the TCU senses over 108% it enriches the mixture so the engine can continue to spool up.  I could get the RPM up to around 4300 and 32 in of manifold pressure.  If I pushed the throttle further the engine would bog down, however the manifold pressure would stay the same with no increase.  Well, if you want more boost you have to have more fuel and with the crack in the hose the three way valve was trying to do its job but the pressure was escaping.

    I suggest every 914 owner check your small rubber hoses very carefully on a regular basis.  This was found on a brand new engine with only 9 hours on it.  It appears to me that maybe the hose ID was too small for the barb fitting it was installed on causing a stress crack.  (see picture.)  My initial troubleshooting was to the carburetor bowl as there was lots of discussion about over torquing them.  Even though the engine ran perfect up to 108%.  But after checking them thoroughly, they were found to be not damaged.  As I said I was fortunate enough to contact a Rotax source with the suggestion I check the 3-way valve hoses.  And he saved the trip.  I actually ended up cutting a 1" piece of airbox drain hose to fix the cracked one.  A short run up proved the repair and all is well now.   

    32681_2_cracked hose.jpg (You do not have access to download this file.)

    Thank you said by: Ken Ryan, RotaxOwner Admin

  • Re: 914 won’t go past 4100 RPM

    by » 6 weeks ago


    That's wonderful news Clyde. Just a word of warning on your hose selection "1" piece of airbox drain hose "  - Make sure the hose is compatible with the fuel that you are using. Its not unusual for a low specification hose to be used in a  "drain"application.

     


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