problem with 1992 honda 250x?

[Joey]

i bought a 1992 honda 250x and yesterday it ran fine then this morning when i first rode it it popped and it wouldnt start and the guy i bought it off of said it was a 2 stroke so i put mixed gas in but then i found out it was a 4 stroke so i drained the gas and i changed the oil and put in a new sparkplug...and it wont start...and when i do get it started it wont run right like when i go in 1st and when i push the throttle in so far it backfires and like dies then when i let off the throttle it ok..i dont know much about quads and i was hoping you could help me