Hi,
For over 60 flights of fun-fly powered by OS91FX I fought over poor fuel flow to carby. I by-passed the original needle valve and took one from OS160FX and screwed it to engine mount. This valve is bigger, so 1/2 to 3/4 opening is enugh for good engine operation. Works well for last 50 flights, and only occassionally requires minor adjustment, like two-three clicks. Because of this my next 90 2-stroke will not be OS. Probably MVVS which including tuned muffler costs about 3/4 of OS (exported from Germany).