250 straight six starving?

[Steve Van Gent]

Trying to get my sons 71 stepside with 250 straight six running. Here is thread to see truck - http://67-72chevytrucks.com/vboard/s...d.php?t=427824

Last owner had 6 in poor condition, bad fuel pump, mating gasket on intake/exhaust burnt out, bad alternator, greasy mess. We started working on this weekend, got running ok, found coolant leaks every where. Bad one, rear freeze plug. So we pulled motor, trying to replace plug-also degreasing motor and compartment.

Here is question- While motor was running, motor idles great, timing is correct, plenty of fuel, new inline fuel filters for catching rust, etc. Motor hesitates and backfires when I try to accelerate. Once I get motor past 2000 RPMS, runs fine. BUt the initial acceleration always results in backfire, trying to stall, hesitation. I pulled carb, it looks fairly recent manufactured carb, and is labeled as a Napa reman carb. Looked clean inside. Now I am stumped, not sure why it is stumbling....any ideas? Seems to have good spark, lots of fuel, no vacuuum leaks, new gaskets on all manifolds.

????