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New fuel pump

in 912 / 914 Technical Questions 13 years ago

  • Re: New fuel pump

    by » 11 years ago

    Here are some of our THEORIES about the reason for the pressure drop after switching off electric pump.

    The diaphragm in the mechanical pump moves back and forth by two springs. The pump shaft indirectly affects the membrane by exposing one of the springs at each pump cycle. (The pump shaft allows the diaphragm to move but do not perform the movement itself.)

    When the electric pump is running, it generates a positive pressure at the inlet to the mechanical pump. The excess pressure keeps the diaphragm in the rear position and probably ceases diaphragm movement completely.
    The check valves are open and fuel flows continuously through the pump without pulsations.

    When the electric pump switches off, the pressure drops. The pump diaphragm starts to move about 30 times per second (depending on rpm) while harmonizing with check valve's movement to work up the pressure. Most likely, it will take longer for the new small pump to restore pump function to generate pressure and flow compared to the older larger pumps.

    As I mentioned earlier, the pump's maximum stroke length is almost the same as the lift of the pump cam. It is therefore probably important that the pump is correctly mounted on the gearbox.
    If the pump shaft is depressed too much when the pump cam stays in the inner position then the pump diaphragm stroke will decrease. (Reduced pump capacity)

    I apologize for bad English.
    Björn
      
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  • Re: New fuel pump

    by » 11 years ago

    There is a fuel pump made by a third party called the Billet Pump (http://www.billetpump.com/). This pump seems much more robust than the pump supplied by Rotax, and has a pressure regulator. I have heard that these pumps are extremely reliable, and hold pressure constant. Looking at the pump, it seems much more robust.

    Does anyone have experience with them?

    Doug
      
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  • Re: New fuel pump

    by » 11 years ago

    Just an FYI,

    In the Installation manual it does say the re-circulation line restrictor is .35mm or .014".

    Actually that is a misprint that should have been fixed. The restrictor should be a #35 Mikuni pilot or slow jet. It is actually based on fuel flow and not hole size from Rotax. The new pump really needs a re-circulation line or it can give erroneous readings and have high fuel pressure.

    I just removed a Billet fuel pump and pressure regulator off a Sport Cruiser that was giving the owner all sorts of grief. I put the new Rotax pump on and he complained of odd readings. As is the case about 90% of the time it was the fuel pressure sender. These senders are wrong a lot more than anyone really knows. I changed that and his pressure went right to 4.2 and with the electric it went to 4.7. That's pretty much right on the money. Before you blame the pump at least change out the fuel pressure sender and rule it out. Put a restrictor in this line to because it will keep the pressure fluctuations down and make the pressure more stable.

    For the people having issues with the new pump make sure your fuel delivery system is really set up in accordance with the Rotax recommendations.
    I have installed a lot of these pumps and not a single person is having any issue. I have one on my own plane. Rock solid at 4.5 psi.

    p.s.
    I now have a used Billet fuel pump and regulator if you want to use it. :S
    Naw, I wouldn't do that to anyone.

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

      
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  • Re: New fuel pump

    by » 11 years ago

    Thanks for the feedback on the Billet pump.

    The restrictor in the return line might be the issue. I will contact Tecnam to find out what size restrictor is currently in the return line, and if necessary will modify the return line to come in compliance with the installation manual with the #35 Mikuni pilot jet. Since the fuel pressure drops when I am climbing and the engine is using the most fuel it makes sense.

    Since I have a manual pressure gauge that directly connects to the fuel system without a sender I am sure that I don't have an issue with the fuel pressure sender. Although I haven't taken the fuel pressure gauge off and checked it for calibration, I feel confident that it is did not break the moment I changed pumps. Before I changed fuel pumps, the electric pump read 0.30 bar when activated before starting the engine, and reads 0.30 bar when activated before starting the engine after changing to the new fuel pump.

    Glad to hear the positive results you have had with the new fuel pump. That confirms what the Rotax representatives told me.
      
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  • Re: New fuel pump

    by » 11 years ago

    Hi Doug,

    It can be the sender. With new pressures you're making the sender stretch its legs more so to speak. I have seen this happen numerous times. Until you try a different or new sender it can't be ruled out.During your diagnosis don't bypass any check no matter how simple an answer. It is usually the simple things and by bypassing a diagnostic check you may look in all the wrong places before you come back to the simple answer. Just try and and see. Nothing to loose and a simple easy fix.

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

      
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