[Yorkie]

I always start with the basics - recheck the valve clearances. Set the timing again. Check the plug gap. The basics. Then I go back through the carb, especially paying attention to the jets and their cleanliness. Finally, do a compression check - you might have slipped up fitting the new rings. If it starts up easily, revs cleanly then you might need to look at things like a slipping clutch. Good luck.