omg-rngy

'03 Suzuki Ozark 250. Put a full tank of gas in it and took it Muddin. Halfway through the day it started backfiring and had no power with anything more than half throttle. If i turn the choke on it clear up a little but still has less power. Please help. Also, this may be related to the issue, but i doubt it. It smokes on start up but usually clears up after a while, but as of late has gotten slightly worse. May just be the oil i put in it. Please help!