Your carb is letting fuel pass when you're not running it. It's a gravity fed system. Probably the float. You can fix it at the carb with a bit of time and $. I'm lazy so when I have this happen, I get an inline fuel valve and shut off the gas when I'm done for the day. A $7 fix. BTW, get a snowmobile version if you can as they're good for all 4 seasons.
Next time you have it in the shop (if ever), have them fix it but I've been using the aforementioned $7 valve for years without any further issue.
You need to drain your existing oil to get that gas out. Once you do that, drive it hard for awhile to burn it out of the exhaust side of business.
change the oil, started ok, idle ok, took it for a drive and took it up a hill and lost power and let off of gas and punched it and took off and ran great, stop and look at something, left it running , got back in and it started missing and loseing power again.