Just getting back to this model airplane thing, after about a 50 year absense. (Played with the control-line stuff as a kid) Like to tinker with engines, so thought that I'd try a "conversion" for my first attempt at RC. Got a 31cc Ryobi, & spent several hours on the engine stand, trying out different props, carbs, reeds, modified manifold, etc.. No matter what I tried (even some 10% nitro glow fuel) performance was terrible. Finally, put a degree wheel on the engine, & checked the exhaust. Couldn't believe the reading. (118deg)!!!!
Bought another Ryobi, & compared the exhaust port timing on the new jug. Was about 138 deg. Still very mild, but a long way from 118 deg. Put the new jug on the engine that I had been running, & gained nearly 700 rpm. Pic included to show comparison. Low port is the lo-perf. unit. Time to break out the die grinder! Geo.K