I HAVE A 1992 KAWASAKI KX 80 DIRT BIKE, AND NEED SOME ANSWERS?

MY KX 80 IS HAVING A PROBLEM AND I NEED TO KNOW WHAT IT IS. I WENT FOR A RIDE AND FILED UP THE PLUG S I WENT HOME CLEANED IT WENT FOR ANOTHER RIDE AND SAME THING SO BOUGHT A NEW PLUG AND THEN TRIED IT KNOW IT WILL ONLY STAY RUNNING FOR ABOUT 2-10 MINUTES IF THAT AND BOGGS OUT... ALSO THE COMPRESSION IS GOT A LOT HARDER IN HARDER EVERY MORE TIME I KICK IT OVER.... PLEASE HELP ;(