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Thread: 914 wouldn't go past 4300 RPM

  1. #1

    Join Date
    Apr 2016
    Location
    Buckley, WA
    Posts
    38

    Default 914 wouldn't go past 4300 RPM

    On a recent trip to Eastern Washington, we departed the Tacoma area on a 2:30 hr trip to Pullman, Washington where we fueled up to go further into Idaho. Throttle going forward during take off only produced 4300 RPM and pushing further caused a bog and decreased RPM. Back at the ramp several hours of troubleshooting with carb bowls and other suggestions from the FBO produced no results. After a couple days of research I again went to the airport to try and figure out the problem. Still no results.
    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 takes 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.

    cracked hose.jpg

  2. #2
    Senior Member Delta Whisky's Avatar
    Join Date
    Dec 2017
    Location
    Amissville, VA
    Posts
    525

    Default Re: 914 wouldn't go past 4300 RPM

    Thanks for sharing this finding. Maybe this could be the start of an "Engine problem and solution" thread. Very helpful.

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