Have you tuned the engine?

What size arm are you using on the servo, how much ATV percentage? My experience with gasser linkages suggest we need to move the carb throttle shaft approximately 80 degree's and the servo near 120 degrees to realize decent resolution and throttle response. Obviously the linkage/bellcrank ratio plays a big part of the final output ratio. You should be using near MAX ATV or end-points with the appropriate servo arm length for the best results.