RE: ys engine surging at full throttle
The problem is, you won't really be able to tell on the ground if there is an issue with blow-by on the piston or the valves and valve seals. They might be fine below normal operating temps, but once the engine is at full operating temperature the metals expand allowing for small leaks to affect the engine's performance. If you can't figure it out, I'd say send it to YS parts and service.