Got about two hours on a new OS 35 AX engine on a Tiger 2 and really like the engine. It has a stagger though after idle and going to mid-range of throttle, as if it is loading up with too much fuel. Tried leaning the idle mixture a lot but it doesn't affect the stagger. The engine normally flies fine, but after a long taxi it might stagger and die just when you are ready to take-off. I have been running it on the rich side to break it in well, but have never had this problem with dozens of other OS engines. Any ideas on resolving the stagger?

By the way, I am using the pressure tap off the muffler to pressurize the tank. No fuel filters. Have backed out the main needle valve to clear any possible dirt. All ideas appreciated.