have you checked you jet screw? it might be too large or too small. i have a dellorto 14.12 carburator w/ a 55 jet (i'm hitting a bit over 30mph). also check that you have a large enough manifold, or else you won't get the fuel into the engine itself, which will slow you down. finally, you might even want to change your piston and cylinder to a larger size. i did that also before the carb change (i had a dellorto 14.9) and gained about 5 mph on that change alone and another 5 mph by changing from a 62.5 jet to a 49. finally, check the timing (something i haven't yet done and don't really know how, though i know it's important). hope that helps.