engine backfires when rev'd really high? on 1990 Chevrolet Blazer

The vehicle is a carburated chevy 350 K5 Blazer
Starts and seems to idle fine, It is set to 8 deg. BTDC. 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
check the distributor cap for a crack. common issue.

All inspections done with the naked eye, Indicate that the Distr. Cap appears to be fine