Hello; I like what Ken says; OS "F" plug is the one I use on my Gee Bee with the Saito 91. There are low speed adjustment screw on the carberator. It is in the middle of the throttle lever. I would recommend that you adjust the low speed by using the pinch test; pinch the fuel line with the engine running, the engine should accelerate slightly before dying, this indicated a slightly rich idle setting to accomodate that extra burst of air when you open the throttle. If the engine just dies when you pinch the fuel line, you are too lean on the low speed and will have to back it out a bit and test again. I usually go one eighth of a turn at a time until it's just right. Like Ken says, the 91 is a pretty big engine to use with that plane, and on pavement it likely will creep forward on idle.