Well it's been years playing with a piped motor, but if pipe is to short high rpm and no needle valve adjustment. It will go lean/rich very quick with one or two clicks of adjustment.. You will want to be able to move approx 1/2 turn either way. Also on the OS91FX you'll want to adjust the low end needle to help transition and also 25% nitro helps clean up the mid-range. Well hope some of this helped..

Scott Anderson
Team Castle Creations
Team Tanicpacks.com