Ken Choo,
Pull out the operator's manual and reset the engine to the perscribed settings per the manual. When my engine was new, I scribed a line on the casing where the factory setting was on the regulator. The setting was 3/4 of a turn from seated, or about flush with the casing body, set the low speed screw 1 1/2 turns out from seated, and set the high speed neddle 1/12 turns out from seated. Am using power master 20/20 fuel, a 16x6 master airscrew prop ,and a custom 3/8 i.d. exhaust about 20 in. long.With these settings and this combo, my ys.91 goes from 2000rpms to 8600 rpms static in about 1 second, has a super precesion idle, and pulls like a 747. Ys says not to rev the .91 past 9000rpm static on their web sight so keep this in mind.
One of the biggest helps for this engine believe it or not is the spinner. If you are running a plastic spinner, throw it in the trash and buy a true turn spinner. I'm running the combo in a h9p-51 and am amazed how much better the .91 runs with this spinner, and also picked up betweem 750 and 1000rpm's more. The true turn is litterly 0 balance and helped the .91's low idle emensly. It acts like a fly wheel and the idle is soooo sweet, can idle for a good minute then roll the throttle to full and instant 8600rpm's, no stumbling, hesation, or stalling. Have run my .91 with these settings for at least 70 flights, and have'nt touched the regulator or low speed screws, and have only had to adjust the high speed leaner or richer 1/8 of a turn.
Hope this helps....good flying.........