Sounds like it may be lean. My ST90 is somewhat sensitive on both needles but I started with them at 1 1/2 on the idle and 1 3/4 on the high. Adjust the idle mixture first and then move to the high needle. The idle mixture affects the high speed just slightly so any time you move it be sure to recheck the high speed needle. If the small screen inside the carb is dirty it can also cause surging so you might want to check that.

P.S. What is venturi size on the Walbro you are using? If the venturi is too large it can cause fuel delivery problems at higher power settings.