New at this site. Looking for a little help. Bought an old 1979 F-250 with a 4.9 six. Most all of emissions gone. Since I bought it has always been weak and 10 mpg at best. Then it started shutting off, then right back on and backfiring. Now when you lightly press accelerator it just surges and when traveling down highway it backfires. Changed ignition module (seems to be frequent) checked wiring harness from module to distributor. Have replaced intake manifold, rebuilt carb, replaced secondary pick up coil in dist., replaced dist cap, plugs, wires. I can spray aerosol (carb cleaner/starting fluid) around carb and no vacuum leaks.
Any suggestions? Thanks in advance.