My OS .46 FX ran great for about 3 months and then started to act like yours..... I had air bubbles between needle valve and carb.... I started by replacing the small nipple fittings on needle valve and carb, still had bubbles. Replaced the O-ring on the needle valve, still had bubbles. Ended up cutting/machining the needle valve off the crank case and replaced it with a tower hobbies .46 needle valve - double O-ring type. No more bubbles and engine has run great since. I myself run Powermaster 20 or 30 % (after break-in) with no problems as of yet..... Not saying its the best for the engine rather just tired of having multiple fuel jugs all the time. I do ensure its just a tad rich though.

Smitty