/6 not wanting to start after capping off vaccum lines.

If you have the 1920 holley the idle mix screw is on the valve cover side of the carb at the base. Turn the
screw counter clockwise to richen.

Check your timing and valve lash! My car was backfiring....turned out it needed a new timing chain! That's most likely your main problem. Basically the valves aren't closing at the right time and are igniting the fuel in the cylinder then thru the valve and intake/ carb hence backfire. There is a link in the Engine FAQ section for the valve lash process.