It may be starving for fuel. I had a 190 that did the same thing. I took the fuel nozzle out of the carburetor and cleaned the holes with a set of tip cleaners. This opens the holes a small amount and the fuel flows better.
I had a B&S on a generator that would only run with the choke half closed. I did the same with the tip cleaners and it now runs just fine.
Hope this helps.