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Dear guys, hope somebody has an idea what might be the problem with my Rotax 912 / 100 hp:
Last weekend I flew  ~ 5h with my Tecnam P92 Echo S equipped with Rotax 912 / 100 hp (~1.200 h operating hours) without any problems. Suddenly I recognized a drop in speed of about 200 rpm for about ~2-3 seconds. I could feel the drop in speed and see it on the tachometer. This behaviour continuously recurs within every ~ 60 seconds. Always for ~2-3 seconds. Next day I checked the engine but founded nothing unusual. The first hour of my flight was okay. But then I had the same strange behaviour. I rather think it is an electronically problem than a fuel or carburettor problem. I think so because of the constancy of the drop of speed about 200 rpm for 1-2 seconds all 60 seconds. I didn't had the confidence (I was too shy) to switch off one of the ignition circuit to check whether the problem is the other ignition circuit. Almost as expected I couldn't repeat the problem on ground later. It looks like I have the problem only after a while (~ 1 hour) of flying. Any ideas? BTW: I am German, so sorry for my English. If something is unclear or unprecise described please let me know. Thank you for your support! Michael

  • Re: Unusual drop in speed Rotax 912 100 hp

    by » 3 months ago


    Don't worry about language,

    you have reached the best forum, now here a lot of expert. you will get the solution very soon.

    Have you synced the carbs,

    Dry Floats weights?

    Have you checked the carb float bowls for debris?

    An engine rpm change could be from several factors. Most are easy to check. Does this rpm change happen all the time, is it momentary or drop and stay t at the lower rpm, does anything seem to make it change, does it happen only at take off or is it repeatable in level flight?

     


  • Re: Unusual drop in speed Rotax 912 100 hp

    by » 3 months ago


    Thx for your quick reply.

    - Yes, carbs are synced.

    - What do you mean by dry floats weights? Couldn't find a reasonable translation. Sorry.

    - No, I have not yet checked the carb float bowls. Good idea.

    Unfortunately the rpm change happens not all the time. It occurred two times after more than 1 hour flight time. After landing everything looked okay. But I don't think it has something to do with the flight altitude. I flew always in low level (2.000 ft above ground). The drop of 200 rpm down is only for 2 - 3 seconds. After 2-3 seconds the engine runs perfect as before for 60 seconds. After roughly 60 seconds again: 2 - 3 seconds 200 rpm less.


  • Re: Unusual drop in speed Rotax 912 100 hp

    by » 3 months ago


    Open the carb bowls and you‘ll find two floats in every bowl. They swim in gas. Some floats seem to soak up some gas and therefore get heavier than they are supposed to be. Take the floats out let them dry properly and take their weights. If too heavy replace them with new ones (you’ll find the proper weight in this forum, was it something like 4.7 grs or so?).

    From your description I don’t suppose you are having a float weight issue.


  • Re: Unusual drop in speed Rotax 912 100 hp

    by » 3 months ago


    @Peter: Thx for your reply. What do you think might be the problem: Around carbs or around ignition circuits?
    What is a reasonable procedure. It looks like I should clean the carbs  and carb float bowls first....
    Am I right?

    BTW: If I accelerate the speed during speed drop down, the engine takes the increase. Could this happen if dirt or water in the carbs is the problem?


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