I'm not familiar with the hitec radio, but if you can, work with the end points and sub trim.

First though, you need to make sure your linkage is set up correctly. At mid point, you want the servo arm to be at 90 degrees to the pushrod/cable. At this same setting, you want your throttle arm to be at mid point and also at 90 to the pushrod/cable. This will give you the best linear movement. On the 4* I'm just finishing, the throttle cable approaches the servo about 15 degrees off centerline. If I set the servo arm so it is 90 degrees to the center line, the my travel is all on one end of servo rotation. This drove me nuts on my trainer.

You put a wheel on the servo, and pull the rod or cable so it just touches the edge of the wheel, that point is where you want the servo arm to point when the servo is centered. You probably don't have the ability to make as much adjustment at the carb, but you want the same relationship, or as close as you can get.

Don