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Hesitation when going to full throttle

in 912 / 914 Technical Questions 11 years ago
I do some maintenance on a motorglider for a friend. It is fitted with a 914 (certified). Ever since this aircraft was new, there would be a hesitation in the engine when going from 100% to 115% on takeoff.

We've asked elsewhere but no-one seems to know why it does it or how to fix it.

So asking here hoping there might be some fresh ideas.

Thanks

Cath

  • Re: Hesitation when going to full throttle

    by » 11 years ago

    A lot is happening as you go to full throttle: At approximately 108% the wastegate closes and the enrichner solenoid closes giving more pressure to the float bowls (and therefore richer mixture).
    -The most likely cause is a slight float bowl leak. The bowls may have been overtorqued at some time or the gasket is not sealing well. Take a good look at the bowls; if the float pins are splayed out the bowl has been overtorqued and needs to be replaced. Make sure you get the new style plug with the o-ring seal as it is better. the torque is only 49 INCH pounds (not foot pounds)
    -Pre-soaking the float bowl gasket in fuel helps to make a good seal.
    Other items to check:
    -Is the throttle calibrated? This is important and often not done.
    -Carefully check the hoses that connect the enrichner solenoid/ram air port/float bowl vent/tee/fuel regulator. Grab the hose and bend/distort it at the point where the it connects to the fittings; this is were it will be cracked.
    -Make sure the airbox/carb rubber is firmly clamped. With a firm pull you should NOT be able to pull the airbox away from the carb.
      
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  • Re: Hesitation when going to full throttle

    by » 11 years ago

    Thanks for the suggestions.

    I completely overhauled the carbies last year including all new gaskets, the o-ring on the plug and rubber parts as a component in the 5 year rubber parts replacement. The float pins are not splayed out. We found that error in another similar motorglider a few years ago resulting in the floats being frozen in place.

    All the hoses of course were also replaced as was the airbox/carbie rubber. None of the old hoses or parts were cracked.

    Throttle position was checked but we might do this again.

    After all this work, there was no change to the hesitation that happens. :(

    -Cath
      
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  • Re: Hesitation when going to full throttle

    by » 11 years ago

    Hi Cath,
    try connecting a MAP gage out of a twin as per the attached instructions; maybe you do not have enough fuel pressure and the carbs are momentarily being blown dry.
    You can get a dual MAP gage at most salvage yards or instrument shops.
    Rob
    fuelpressure.pdf (You do not have access to download this file.)
      
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  • Re: Hesitation when going to full throttle

    by » 11 years ago

    Thanks I'll try that. I have a dual needle gauge somewhere out of a Baron I think.
      
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  • Re: Hesitation when going to full throttle

    by » 11 years ago

    Had the same trouble it ended up being the pressure regulator so the duel gauge should show that
      
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