I tried the recommended O.S. carburetor today and unfortunately it didn't help the engine perform any better (engine continues to run ~ 5100 RPMs). While it did provide more fuel to the engine, I had to adjust the needle valve to 3/4 of a turn open to get the engine to run properly which is an odd setting for a needle valve. Furthermore, the throat on the O.S. carb is significantly smaller than the already small EVO carburetor. Interestingly enough I gave the evolution carburetor a thorough cleaning and can now get the engine to run rich with the stock carburetor. Oh well, only thing left to do now is maiden my Ziroli Dauntless... As long as the engine is reliable I think things will be okay. The Dauntless doesn't need to scream around like a fighter.