Ok, so my DT400 suddenly wouldn't upshift. 1st and neutral only. No external reasons. Split the cases and found a complete gear tooth laying inside. Along with a partial tooth and chunk of the case. 3rd gear was missing the partial tooth. Found the busted case near the tach drive. No gear missing a complete tooth.... I can only assume the previous owner left it behind for the Tooth Fairy which cost me $55 dollars for a new 3rd gear and a gasket set. Witness marks on the shift drum stopper plate tell me one of these pieces had settled in next to the spring detent. It would rotate in one way for 1st gear but not the other way for the remaining gears. I hope my theory holds true because I don't want to go back in there.......