fbpx

 

Tried posting this on an old thread titled “drop rpm” yesterday thinking it would bump that thread to the top of this forum but it did not, so starting this new one.

I want to ask what folks think may have caused my 100hr TTSN 912UL engine built in late 2018 to drop a needle width in rpm and immediately recover. Happened twice, about a minute or so between occurrences, shortly after takeoff after setting power for cruise.  Heard it, and confirmed it on the tach. Just a blip of a drop, more like a stumble, duration less than a second, couldn’t have been more than 100 rpm drop, then returned immediately to 5,000 rpm. Cruising along on a clear sunny morning, OAT of +8C, clear and sunny but high humidity conditions.  First time something like this has happened since 2019 when I finished the build (snugly cowled Zenith 701, tractor configuration, K&N filters) and I fly year round.  No perceptible change in any engine gauges, including the fuel flow gauge, when I glanced at it the second time it happened, but rpm had recovered by then. Oil temp around 180F at the time. Only thing I could come up with was possibly carb ice, so after the second occurrence I advanced the throttle to 5,500 rpm. It did not happen again so stayed at 5,500 rpm. Oil temp gradually rose to around 200F and remained there. The flight back home was uneventful at 5,000 rpm cruise power, OAT had risen to +15C.

Aircraft is hangared year round and flown every two weeks on average. No abnormal issues noted during run up check. All three fuel sumps were drained prior to flight as for every engine start. No water or debris present. I don’t even pull the prop through on preflight without draining the sumps first. The carb bowls were removed to replace the floats a few months back and were clean and debris free. Carbs mechanically and pneumatically synched last November during the 12 month inspection, Rotax $ plugs cleaned and reinstalled with the paste. Flown several times since the inspection, no problems. Same “regular” ethanol-free Mogas, AKI 87, from the same source, since first flight of the completed aircraft. Exhaust system is not “wrapped” and is the same age as the engine. All fuel lines are rubber with 2.5 years in service. Everything under the cowl still looks new. Any thoughts?

  • Re: Carb ice symptoms

    by » 10 months ago


    It doesn't sound like carb icing, that is a much slower process to form and clear. I would guess that something momentarily blocked the main jet. I would drop the float bowls and take a close look. I had a similar issue that turned out to be debris in the one of the bowls, it looked like a tiny bit or rubber, probably from one of the supply hoses.


  • Re: Carb ice symptoms

    by » 10 months ago


    My 912ULS went through an extended period of  momentary "hesitation" . Seemed to happen over "Tiger Country" during extended climb out, so probably well above 5000ft.

    Tried a few things, including using AvGas, without finding the cause.

    Problem (if there was one) disappeared.

    I think we expect flawless, vibration free performance from what is actually a pretty basic engine - the odd hesitation while concerning (& we should investigate) is to be expected.

    I did another 600 hrs in that aircraft/engine without any issues.


  • Re: Carb ice symptoms

    by » 10 months ago


    HI jim

    Not enough information to really make guesses, that is the nature of spitballing problems I think.  You might consider the fact that at 8C temp makes the air pretty dense.  As you noted once you got some heat in it there seemed to be no problem.  Remember that the carb is lean at the 75% power settings, 5000 approx.  If I was to guess it seems to me that perhaps you were set too lean for these conditions.  if this is normal for this time of year there consider moving the clip on the needle one position to richen up that midrange.  I know it does wonders for high mag drops in similar conditions.  

    Just my thought.

    Cheers


    Thank you said by: Jim Isaacs

  • Re: Carb ice symptoms

    by » 9 months ago


    Jim what you describe is exactly what I have experienced on a number of occasions with the 912ULS in a Kitfox S5 with the K&N filters i.e. drawing its warm air from the back of the motor. It only happens in conditions where one would expect carburettor icing i.e. high humidity combined with low engine temps. 

     I have concluded that this momentary hesitation you describe is ice being ingested into the engine. Increasing power, and thus the engine temps, has always eliminated it. However I am installing a vernier cockpit controlled cowl flap so that I can increase engine temperatures (and thus the under cowl temps) whenever icing conditions seem likely. I hope this helps.  


    Thank you said by: Jim Isaacs

  • Re: Carb ice symptoms

    by » 9 months ago


    Thanks Kerry. I block off about a third of the surface area of my oil cooler and remove the scat tube and shroud for my winter flying. Oil temp still won’t exceed 210F even during extended climb out at 5,500 rpm.  This past weekend I dropped the float bowls, checked the new floats, inspected for debris or anything else amiss, nothing found. Vent lines clear and properly routed. Ran a thin wire strand up the main jet, all clear.  Checked throttle rig, all good. Checked the ignition module wiring, ignition switch and wiring, all clean and secure, including the grounds…checked the plug wires and caps, all good.  Ground run, power checks, all good. Took her up for a flight, all good, engine ran flawlessly.  Takeoff temp 18C, dew point 12C.  Could not duplicate those two bump drops from last flight.  On a totally unrelated note, doing some slow flight practice I noted the winds at 2,000 AGL were quite strong. Snapped a picture.

    32200_2_F2E710F9-EC27-4D63-8C56-BED1177EDEC5.jpeg (You do not have access to download this file.)

You do not have permissions to reply to this topic.