318 will not start

Had a similar issue years back with my 87 D150...after replacing everything, and I do mean EVERYTHING (fuel pump, fuel filter, orange box, ballast, plugs, wires, cap, rotor, distributer, coil, and carb) it still didn't work....then I found 1 small vacuum leak at the brake booster, fix it and it started right up and ran like a champ.....I felt like a serious idiot after that, lol.