What brand of prop are you using? I have tested many props on gas engines and I have found out that in the 18" range you will get more thrust and rpm from an APC brand prop than others. Here is an example of the props tested on a Brillelli 25Gt engine I have.

MAS 16x8 prop: 8200RPM Thrust: 12lbs
MAS 17x6 3 blade: 8000RPM Thrust: 12.3lbs
Dynathrust 18X6 prop: 7800RPM Thrust: 12.4lbs
APC 17x6 prop: 8800RPM Thrust: 14lbs
APC 18x8 prop: 8000RPM Thrust 14.3lbs

The APC props cost a little more, but you will get more RPM and Thrust out of them over other brands.

Thanks