with new plugs it runs for a little while

i have a 1996 camaro z28 with the ls1 in it i changed the spark plugs in it and it ran fine for a little bit but now it wont start but it will crank over and it back fires when it tries to start what can it be ? I changed the spark plugs they had carbon on them, after changing them it started right up drove it about 20 miles turned it off and let it sit for 2 days and now it wont start up. But it will crank up and it backfires when it tries to start do u think it could be the timing, or the opti spark that went out?