It's only a 30 size motor, and it's a 4-stroke at that. I can't believe you're turning 9x6 props. [X(] I had no idea these engines would turn that much prop. A 9x6 is a 40 size 2-stroke prop around here.

Why not try some 8x6 3-blade props? The extra blade will help with the flywheel effect KMOT was talking about. That should slow it down at idle too. The smaller disk will not move as much air at idle and the 3-blade is less efficient to boot.

Lots of good reasons to run 3-blade props. You're not looking for more performance anyway. The Tri-Star is just a mule to break in these engines. Who cares how fast it is or how much vertical it has? Your just putting time on the engines. The 3-blade props should help accomplish your goal of slower taxi speeds and lower idle.