Carb ice symptoms

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.

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.

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

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.  

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.