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Rpm drop and change after takeoff

in 912 / 914 Technical Questions 6 years ago

  • Re: Rpm drop and change after takeoff

    by » 6 years ago

    I think that Kiviadt got it

    Aerodynamic definition: the branch of mechanics that deals with the motion of air and other gases and with the effects of such motion on bodies in the medium.

    The vent line issue is a good example of it

    The better way to understand the vent line issue is to watch the video
    https://www.rotax-owner.com/en/videos-topmenu/rotax-minute/454-carb-vent-lines
    Thank you said by: Daniel payne
      
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  • Re: Rpm drop and change after takeoff

    by » 6 years ago

    Virtually all of the 912 installations I have seen have metal platforms or bowls underneath the carburetors to both catch any vent line drips and insulate the carbs somewhat from the hot exhaust pipes. Some of these platforms/bowls have their own overboard fuel drains and some do not. ROTAX refers to these bowls as "drip trays".
    Thank you said by: Daniel payne
      
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  • Re: Rpm drop and change after takeoff

    by » 6 years ago

    The Hose is a Vent Hose meant to equalize the pressure in the Carb Float Bowl with the air in the Carb Intake. (Ambient Pressure)
    It is way more common to get a drip when parked for a long period than you ever will with the engine running.
    Terminate the hose above the Carbs and you will never get a drip from the hose.
    It might pour out the Carb Throat, But the hose won't drip!

    If you are a perfectionist, terminate the vent hose with a small fitting installed in the end cap of the Air Filter.

    CarbVent.jpg (You do not have access to download this file.)

    Bill Hertzel
    Rotax 912is
    North Ridgeville, OH, USA
    Clicking the "Thank You" is Always Appreciated by Everyone.

    Thank you said by: Andrew Dunning, Daniel payne
      
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  • Re: Rpm drop and change after takeoff

    by » 6 years ago

    Bill -- I have considered a carb vent hose rerouting similar to your photo above. However, I have got to think that the air pressure inside the filter (when the engine is operating) has to be somewhat less when compared to the ambient air pressure at the end of the simple vent line set-up. It seems to me that the restrictive filter media would cause the filter's internal air pressure to be lowered vis-a-vis cowling internal ambient air pressure. As a result, the carbs would run slightly leaner (??) than with the simple vent line? Your thoughts? -- David
    Thank you said by: Daniel payne
      
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  • Re: Rpm drop and change after takeoff

    by » 6 years ago

    Actually, the carbs will run slightly LEANER with the vent connected to the filter.
    The filter Slightly lowers the pressure in the Carb throat, and if the float chamber is still at the Higher Ambient pressure outside of the filter, MORE Fuel gets sucked in.
    If you connect the Float vent line to the internal pressure of the Filter, You cancel out the effect of the Filters restriction.
    The mixture will remain constant as the filter gets dirtier and restricts more.

    That is why I added the "Perfectionist" modifier to my statement.
    The Carbs are designed with the assumption that the Filters are brand New and produce No restriction whatsoever.
    We know that this is not true, but it is what is assumed in the design, as the filters are out of the control of the carb designers.
    So the Float bowl "Wants" to see the same pressure that is in the front of the throat of the carbs.
    The Filter lowers this pressure and causes an imbalance in the pressure.
    This imbalance is usually small enough to ignore especially with a new clean well-designed filter.
    If you want to be the "Perfectionist" attaching the vent to the filter balances the pressure "Perfectly".

    Bill Hertzel
    Rotax 912is
    North Ridgeville, OH, USA
    Clicking the "Thank You" is Always Appreciated by Everyone.

      
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