My 1990 G20 Chevy van with a v-6 wont start.?

I have replaced the alternator, ign module, emi, and the thing that started all of this the front mounting embly. I tested the map censor to cover all my bases and it was fine. The ign module tested bad as well as the emi module. I have a spark with my plugs but the engine does not start when I turn it over. I got it to run for a while but had to put gas to it to keep it idling so I figured it might be the map censor but no it tested fine. I replaced the fuel filter hoping that would help and it hasn't helped either. I get feul when turning it over but it fails to fire. I suppose it could be the teeth on the starter but I have to take it off to check. Is there anything else I should check before I scrap this thing?