I just picked up a 2010 Polaris Outlaw 525 from my cousin. It's been sitting for about a year. The thing started right up ran good at idle. riding around the yard It seemed to be ok.Ran it a bit more. Got on it and after letting off the throttle it's backfiring and seemed to bog a bit at slow speed, I changed the plug ( looks like it's running rich). Drained all the gas, including the carb. Sprayed carb cleaner in it. Changed the oil. I'm stumped. Any input would be appreciated.