Need Help!

[quadriderkfx400]

I was riding my Z-400 the other day on a gravel road doin about 1/2 throttle 4th gear when all of the sudden it died. We took it back to the house and looked at it and realized that my oil was filled with gas. So we changed the oil and it wouldnt start. Then the next day we checked the oil and it was filled with gas again, so we changed it and again it wouldnt start. we checked the spark plug and it wasnt sparkin so we bought a new spark plug and still no spark. I dont know what to do please help. Thanks

P.S. if I shake it and try to start it sometimes it will fire up and idle for about 3-5 seconds then backfire and die.