so my 86 d100 (carbed) has had a miss since day one....not noiceable until at idle

has new dizzy,plugs wires, fuel pump etc...

whats the vacuum supposed to be on a 318 (have vac tester)

only thing i can think would be vac leak or carb issue

stock motor minus headers and chrome air cleaner

has stock 2 barrel

 

It sounds like a timing issue. It could be too far advanced, so when you're at idle with high vacuum it would be even more so.

Here's a long shot, but are you losing any brake fluid? Sometimes the master cyl can leak into the booster and the fluid gets sucked into the carb causing a miss.

 

16 to 20 inches at idle is good, but the number is not as important as the movement. It should be steady and not jumping around erratically.