Unusual drop in speed Rotax 912 100 hp

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?

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.

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.

@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?