Michael,

Throttle response of the engines in our toy airplanes (both gas and glow) is far from linear. This is the problem you're describing, right? Have a look at post 8 in this thread:

http://www.rcuniverse.com/forum/gas-...-dle-30-a.html

Alternatively, dial-in some expo on the throttle channel on your radio to make the response somewhat more linear. Some radios even allow to set a multi-point curve to get even more accurate response. Either way, you want less servo movement at the low end of the throttle stick and much more movement at the top, which is basically what the linkage arrangement in the above link is doing.

Hope this is helpful.

Ken