I had that same problem before with a cracked carb. It's a good thing that I saved the parts from another engine that seized up on me. Oh, what happened was that my carb was set too far back, and the part that goes on the intake tube must have been cracking. A piece of that broke off and went inside of the cylinder, and seized up the engine. What's even odd was, that when I took the engine apart, I was able to slide the piston out and that part came right off. Hmmm, I managed to get another 200 miles out of it. How? Well, that part that broke off was curved, and it landed inside of the cylinder when the piston was up, and the curved part rested perfectly on the curved part of the piston and cylinder wall, and it basically 'wedged' itself in place. It just slightly nicked the ring groove and a hhhhhhhhhhhhhhhairline scratch on the cylinder wall. so I removed the ring, and took a burnishing file and re-faced the groove where it was nicked, inspected the ring and put it right back on. For the cylinder wall, 1 tiny ding spot.. Took some wet/dry sandpaper, 1000 grit and just went over that ding so the raised area was flush. For that hairline scratch, I left it. I re-assembled it and it fired right up with no loss of power. Now, what killed that engine was the replacement carb, from another earlier engine. I put it on, and it ran well till it started to run too rich. I was on this road, running at 20 mph for 4+ miles, and felt my engine surge and felt the engine was hot. I immediately pulled over and the engine quit, then went to a gas station so I had a safe place to work on the engine. I popped the head off and OMG! The top of the piston was melted! So, another one to the bone yard. Then with a new engine, I got about 300 miles on it before it started to act like it was running rich. I disassembled the carb, and saw that the float was not floating in the bowl. I inspected it and saw a crack. Now it made me curious, and needed to find another float. I decided to snag one from the melted engine, and to my surprise, that was the cause for failure on the meltdown, a cracked float. Now, where was I going to get another float? Then I remembered about the carb that had a cracked tube. Yes! It was perfect, and then put it in my carb, and my bike ran fine. So, what I have been doing is stocking up on some extra parts for the engine. Especially the float as its cheaply made.
