1994 wrangler won't start sometimes

i just bought a 1994 jeep wrangler 2.5 5 speed. sometimes it won't start, it doesn't matter if the engine is warm or cold. sometimes it will fire right up like normal, and others it will not. it will just crank and if i let it crank long enough it will backfire. please help me. any suggestions. please email me at matthew_a_wilson@dresser-rand.com