Engine backfires when there is a heavy acceleration on 1990 Chevrolet Blazer

The vehicle is a carburated 1984 chevy 350 K5 Blazer
Starts and seems to idle fine, (It is set to 8 deg. BTDC) (And the Distributor Cap seems to check out ok, no visible signs of cracks). When you first start to rev it up it acts ok. When you rev it higher or get on the accelerator it stumbles and there is some backfire. What causes this? And what should i be checking for?

Asked by for the 1990 Chevrolet Blazer
You mention checking the distributor cap for cracks, but you need to check for carbon tracking inside it too. I'm suspicious you may have sparkplug wire problems, that is, high resistance or leakage. Check the resistance of the wires (should be about 5kOhms per foot) and that there are no places where the wires may be arcing to ground. You may try spraying water from a mister bottle at the cap and wires to locate leakage or arcing, or view the engine at night while having someone in the driver's seat, foot firmly on the brake, loading the engine in gear. You may be able to see the electrical discharge from arcing or leakage that way. These problems usually occur when the engine's under load.
It's possible you could have valve spring problems allowing the valves to stay a little open sometimes which would cause some nasty backfiring and erratic running, as well as being a danger of a valve keeper coming apart and allowing a valve to fall into the cylinder destroying your engine. But most likely it's ignition problems so check that first.