RPM drop while warming up the engine...

Thank you Peter,

I'm not aware and understand this icing phenomenon.

How do you explain that after turning the engine off and restart immidiate after, RPM become ok, while temp and humidity are the same?

Best regards,

Nadav Barak.

Nadav, you might want to check out this article about Carb Ice.  Unfortunately, the chart (supplied by the FAA) uses ºF instead of ºC, because we're one of the last three nations left in the world that are still using archaic measurements...

Note that carb ice can form anywhere from 20ºF (-7ºC) to over 90ºF (32ºC). 

And even though I live in Texas, where our summer temps average in the mid-30ºC range, I know from personal experience that carb ice can form when the chart shows it can...  The Stromberg carb on the O-200 in my old Citabria would ice up almost every time I reduced the throttle below cruise power on those hot, muggy Texas summer evenings when OATs were around 30ºC.  You quickly learned to apply carb heat before reducing throttle in that airplane.

Hope that helps.

Heat Soak.

When you run the engine varius parts heat up faster than others. When you turn off firstly the proces that causes carb ice stops, then the heat in other parts of the engine moves around equalisin over time including heating up the carbs.

My best friend , and wife had an engine stop twice with carb ice, both times after a couple of minutes it restarted and ran fine.

I have had carb ice happen on a 25 degree c day.

Here is a metric chart also

I note that you have only 250 hours on the engine.  Have you replaced your original spark plugs yet?  If they are NGK verify the spark plug gap and check for signs of debris on the ends.  If they are the Rotax plugs you can still check them and clean them according the Line Maintenace manual.  Depending on your fuel you may have contaminated plugs.  Auto fuels can contaminate your plugs with carbon, depending on your prop loading it may occur relativly fast.  Avgas wth lead can contaminate with lead deposites, the results are somewhat the same.  In any event  if they have not yet been changed now would be a good time.  

I read the documents you mentioned and watched several videos regarding carb icing.

now I understant the phenomenon.

from some reasson our Rotax specialist says it is not carb icing.

I will continue make tests and report with the results.

Nadav.