Check your vacuum guys.

If you are having a problem where your engine starts fine and idles fine, but backfires when you give it some gas and stalls with no power, check the vacuum. It doesn't take much of a vacuum loss to do this. The book vacuum range for my engine is 16-19, and it was coming in at 15 to 15 1/2. Enough of a loss to give it no power and backfire up through the carb and stall the engine. I tried everything, full tune-up, new fuel filter, distributor, timing. I finally hooked up a cheap $25 vacuum meter to it, and it read low. I replaced the carb and egr gaskets, re-tourqed the intake manifold, and put some thread tape on a port threaded into the intake, and fired her up. Wallah!! 19 on the gauge, and just turning it over I knew from the way the engine sounded that the problem was a vacuum leak.

Simple. A lot of head scratching went into this. Don't let this happen to you!