Set the position of the throttle arm on the carburetor so that 1/2 throttle opening has the throttle arm pointed straight down. You should then have equal travel from there to both full idle and full open.

Set the servo so that 1/2 stick has the servo arm at exactly 90 degrees to the axis of the servo. Adjust your pushrod to maintain those relationships. Now adjust your full-throttle endpoint and idle endpoints in the transmitter so that you don't stall the servo at the extremes. That should give you the travel adjustment you wish. A little bit of fine-tuning of the pushrod geometry should let you dial it in exactly.