Been using my Ulmer clutch kit for a few years now. First on my Vector, then transferred it over to my Nytro. Great performance throughout the powerband and no more slipping belt, saving me from buying replacement belts. The best part about buying the kit is the service afterwards. When I transferred my kit over to my Nytro, I simply gave Ulmer a call, and he helped me set up the weights and settings...worked great.
You can't go wrong with Ulmer's clutch kit.
Check over at ty4stroke and see the great reviews there.