blown motor,

yea probably not worth the money to bigbore it if its only going to be a backup. That cylinder looks nasty man. I would say it may have stressed the rings quite a while ago when you had the leaner jetting in it. This would make sense that it broke right after stopping then as well, because the hot/cold/hot cycle would've put added stress on the metal, making it more brittle. It probably then broke and caught the port, making your day hell and your cylinder junk.
 

I had the same thing happen to me. 4000 miles with the pipes on it and never a problem. Now look at it... BTV is sending me cyl/head/piston. I got all new rings and a full upper end gasket kit from Commetic. 3 stock head gaskets 100.00, stock base gasket 38.00.... Full upper end gasket kit from Commetic includes intake/exhaust/all 3 head and base gaskets. The entire kit is $60.00. I hope the work good. Have a look at your twin squeeky. I actually did not know I hurt the motor this bad, I smashed in a spark plug about 30 miles from camp. I figured I sucked a reed stopper in and spit it out. So, I put in a new plug and rode it like I stole it back to camp 30 miles at 8500+ and over 90 mph on the plowed roads. She only backfired once but ran fine. I got on a spare sled and we rode till 1:00AM. Started this sled up in the morning to load it on the trailer and she ran fine again. Not until I pulled it apart did I know what the extent was. Sweet Yamaha always gets you home.

http://www.totallyamaha.com/forums/showpost.php?p=63467&postcount=1

Doug
 
that looks exactly like what happened to mine, my spark was smashed in and chewed up, i also rode it back to camp but mine backfired constantly on about a 25 mile ride..
 
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