Recently installed the treats 70cc Malossi kit on a 83 Yamaha QT50. I followed the instructions at http://qt50.net/yamaha-qt50/installing-a-top-end-kit-on-your-yamaha-qt50/
Notably, I spent time chamfering the sharp edges on the cylinder ports and then honing the cylinder with a 350 grit followed by 600. The piston moves incredibly smoothly when installed and I've shimmed the plug to ensure appropriate clearance.
However, the piston catches when bottomed out in the cylinder just about as it ready to change direction and move back up. It's baby smooth until I hit that point, after which it takes some force by hand to push it.
Inspecting the cylinder I can confirm that the piston rings have fully cleared all but one of the ports at that point. I've tried chamfering that port twice more without luck. I see no scratches on the piston or cylinder wall that would indicate any rubbing.
I'm looking for additional suggestions. A little afraid to just try firing it up before I get this ironed out... Thanks all!