I had a YZ250 that I used to race in Enduro's and hare scrambles. We put a Big (cant remember weight sorry) brass flywheel weight on it. It made it pull well down low and was much harder to stall. It would claw its way up hills and drop down to near idle without stalling. It was fast off the mark at MX aswell, it seemed to hook up better. I have riden an adventurised XR600 with a big flywheel also. It was like a tractor and smooth as a babys Mind you when you shut off the throttle that flywheel wants to keep on spinning, so the engine wont slow down as quickly. I guess your familiar with that riding trials bikes