1987 suzuki

got a 1987 suzuki intruder vs700gl* when i start the bike it idles at normal rpm but with backfiring. it does that for 5 to 10 seconds then it revs up to something around 4 to 5000 rpm and stays there or moves radically between 2 to 5 until i shut down. i got the bike and it didnt even crank...