02 250 starts and then sputters out

[Demon49]

I got a 02 gz in November. It had a rare miss to it but I didn't think anything about it. I rode it for about 65 miles and it sit for about a week in the cold. I rode it a few more times until it ran out of gas. I didn't know that your supposed to use 91 so I only put in 87. I ran it for a few days and it started dying on me after about a mile. I let it sit for about 6 to 8 weeks during the cold. I got it back out when it got warm enough. Now on first start I can ride about a mile then it sputters and dies and won't start back again for several hours. I've changed the spark plug and drained the gas and put fresh 91 in it with carb cleaner. It still doesn't run after warming up. What could be the problem?