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On first flight of my Sinus Motorglider, the engine repeatedly surged on climbout. The RPM varied between 5400 and 4200 without any input, with a period of about 2 seconds. It was smooth, like I was moving the throttle. Actually reducing the throttle below 4000 stopped the surging, and I made a normal landing.

I subsequently conducted fuel system checks and verified a clean gascolator screen and 20 GPH gravity fuel flow. I did see air bubbles coming down the clear drain tube, but they stopped after about a gallon and did not occur in the next gallon. I found no other problems, including checking the variable pitch prop control, which remained in the minimum pitch position.

The same surging occurred in a subsequent ground after about 45 seconds at 5400 RPM, matching the first flight experience.

I also have had some overheating - CHTs both high and a boilover at shutdown. I had attributed this to a too-long taxi test and too-long ground running on a hot day. Overheating did occur in the ground run mentioned above.

None of the experts I have talked to can explain this, or offer a connection between the two problems - except maybe that the engine is somehow running lean....

Anyone else come across this?
  • Re: 912 ULS Overheating and Surging

    by » 9 years ago


    check the float bowls, you may have contamination from the new fuel system.

  • Re: 912 ULS Overheating and Surging

    by » 9 years ago


    Sure sounds like a fuel delivery or carb issue.

    Along with Rob's suggestion look for an air leak that may be affected by vibration or movement within the engine compartment. i.e. a crack in the rubber carb flange that gets larger under power.
    I would take the fuel issue a step farther and check fuel delivery at the banjo bolt on the carb crossover tube and pull the banjo bolts off the carbs and make sure both flow fuel well and there is no debris in the banjo bolts. Maybe take the top of the carb and check for the diaphragm alignment and make sure there is no issue with the rubber.

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


  • Re: 912 ULS Overheating and Surging

    by » 9 years ago


    My Sinus began losing power during takeoff but would run fine at reduced RPM. Fuel flow tests would sometimes show full flow and sometimes just a dribble. I suspected contamination in the tank(s). After much checking we removed the fuel line quick disconnects at the wing roots and found them to be failing. I eliminated the quick disconnects and now the engine runs flawlessly.

    I realize I'll have to completely drain the tanks to remove the wings but that's small price to pay to keep the engine running!

  • Re: 912 ULS Overheating and Surging

    by » 9 years ago


    Hi Dan,
    I have had the same problem with the quick connectors of my Sinus. One of them blocked the fuel delivery of the left tank and the return line pumped all of the fuel of the right tank empty. So the engine stopped with the left tank full and I was forced to make an emergency landing (luckily I had some 6000 hrs gliding experience).

    Pipistrel couldn't diagnose the problem for me so I was on my own. Some day I noticed when emptying the tanks that one tank was significantly lagging behind the other one. Conclusion: it must be the quick connector of the left tank. As you did I removed the quick connectors all together and the problem was solved.

    Apparently the rubber seals in the quick connectors were the cause when using fuel with an relatively high ethanol content. Some years ago they published an SB urging to install new type of quick connectors.

    Greetings Jan

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