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The 912ULS that is installed in my 2006 Evector SportStar (560TTSN) has suddenly started developing problems. The first problem happened while flying when the engine suddenly lost power for a half-second or two and then power returned to normal. The culprit was first thought to be the old cork gaskets in the carb bowls. They were replaced but the problem continued on the next Functional Check Flight. Then the carb floats were discovered to be decomposing and small flakes of float material were clogging both jets. Idle jets were blown and cleaned and both carbs were tuned and balanced, but a compression check was also performed and the #4 cylinder was only 66/86. Start and ground runup was normal. Climbed up to 7500 over the field, with all normal indications except the oil temps straying into the yellow side above the green band. When retarding the throttle to idle for the descent, noticeable vibrations were felt in the 3300-3800 rpm range. Above and below that, the engine seemed to run just fine. Upon landing, and the throttle retarded to idle, the rpms dropped to 500 and the engine almost conked out. When I added power to keep the engine alive, the engine ran very rough until the rpms increased above 2000. We are going to look at the #4 cylinder tomorrow morning, but just wondering what the collective brain trust might think. Been flying the airplane since 2015 and never had any problems, My background is retired USN and Southwest Airlines. Thanks in advance!

  • Re: 912ULS Runs Rough at Idle

    by » one month ago


    You mentioned that the carb floats "were discovered to be disintegrating" but I see no mention that you replaced them.


  • Re: 912ULS Runs Rough at Idle

    by » one month ago


    They were replaced (for both carbs) immediately. 


  • Re: 912ULS Runs Rough at Idle

    by » one month ago


    If you have had contamination in the float bowls then it is possible that it has entered the idle circuit. There are a number of very small drillings that you can see in the venturi downstream of the butterfly valve. It would be worth cleaning these out. I would just use a can of carb cleaner and some compressed air, be careful not to use anything that would get stuck in them!

    I have had an inlet valve go bad that resulted in rough running although it didn't really affect the idle. The cylinder heads are relatively easy to remove and that would allow you to inspect the valves in #4 if the carb cleaning doesn't help. Opening the inlet valve on mine revealed an almost razor sharp edge to it, it was as though the hardening had failed.

    I have a Eurostar and an extended climb in the summer results in oil temps into the yellow band.

    Good luck!


  • Re: 912ULS Runs Rough at Idle

    by » one month ago


    OK, this website has already paid for itself! Watching the videos on balancing the carbs showed me something - the throttle cables need to be safety wired in-place where they join the carb. After the balancing, the mech (and I) forgot to re-safety wire both throttle cables. On the subsequent PMCF, both cables popped out their housings when climbing at full power. Not a big deal until you throttle back to idle. On the descent, the windmilling prop kept the RPM up. But on the landing rollout, both carbs were completely out of synch, hence that low RPM and rough running engine. Yesterday the cables were safety-wired and the carb bowls dropped one last time for inspection and the jets cleaned out again. We also did another compression check on the #4 cylinder - 74/80 (well within the normal range), bore-scoped the cylinder and everything looked normal. High and low power checks went normally, cowled up the airplane, and flew the PMCF in which she passed with flying colors. 

    The first picture shows the safety-wired throttle cables. The second is the decomposing carb floats (14 years old - the original culprit along with the old cork carb bowl gaskets). Third is the carb bowl - don't forget to clean out that jet in the corner!

    Thank you to Kevin and Ken for your responses. Solving this issue has been a helluva education and thankfully nothing too terribly expensive. This website is a great resource and something I will be utilizing a lot in the future!

    Derek

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    Thank you said by: RotaxOwner Admin

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