I have an 89 w250 with a 318. It will start and run for a few seconds and then stall. If i give it gas while it starts for the few seconds i can keep it running but while im holding on the gas it will backfire periodically and if i let off the gas it stalls immediately. I have been doing a frame off restoration on this truck so there is tons of new parts but I'm curious to see what you guys think and I will say rather I have replaced it yet or not.

 

Start with making sure timing as spot on.

 

I got it running good, I had a vacuum leak or two that I had over looked before