rotax 912 uls age2002

It's a long shot, but have you checked if the choke is not stuck? If it's stuck, the engine works well during warm up but then when you try to rev it up it stalls. It's worth checking travel and cables.

Yes, I checked it, it's correct. If I pull the choke it accelerates a little more, but then the RPMs drop.

Low wing? As I've posted several times on here before, check for an air leak in the line somewhere between the fuel tank and the input of the fuel pump. Diaphragm pumps will pump and maintain pressure with air just as well as fuel. A leak will supply a mix of fuel and air to the carb bowl. As long as enough fuel is being delivered, the engine will run. But when demand exceeds supply, it won't. The pumping capacity of the mechanical pump depends on rpm. Hence the problem will not appear at idle when the demand is minimal or at full power when the pumping is maximal, but somewhere in between. A clue can be had with use of the rat-tat-tat kind of electric pumps. Engine off on the ground, turn on the pump. It will rattle as it builds pressure, then mostly stop. If it doesn't, you got an air leak. The leak is generally not obvious because it's a suction leak, not a pressure one. After 2 years of trying to find the root cause of this problem, I tracked it down to a bad o-ring on the shaft for the tank selector valve. 

Check for metal in oil filter/magnetic plug. Ive seen some engines doing the same, found camshaft and lifters were toasted. Run perfect on idle but cannot take off.

My 2007 912ULS high wing had the same problem twice. Each early spring. I fly in the Pacific Northwest and use 91 Octane no ETH car gas. I replaced fuel lines, throttle springs, fuel filters, and cleaned carbs. Nothing worked until I dumped the fuel and put in 100LL. I suspect it was winter blend car fuel causing some sort of vapor lock when the weather started to get warmer. Now I fly with a 50/50 blend of car gas and 100LL. No more problems for years,