I'm having troubles with my OS .91FX similar to others I've read here. An air leak sure seems like a possible cause. I've ordered a new O-ring for the needle-valve, and also ordered a K&B remote needle-valve if the O-ring doesn't help.
I also want to put some silicone around the base of the carb and screws like I've heard about here. Concerning the carb, would I remove and discard the gasket that's there and replace with a bead of silicone, or just put the silicone over top of the gasket? Also, will doing any of this void the warranty?

Thanks for the advice, B-ski

In doing this, make sure you don't allow any silicone to get internal to the engine or to plug any passages. Use it sparingly. you probably should use the silicone over the old gasket.

Jim