What will cause slight miss idling on a 65 170?

i would guess carb, i have the same thing going with my swinger

get the engine up to temp, then rev the engine to about 3000 rpm and pour a small amounts of water down the carb. its gonna stumble and then clean out, pour some more. I usually do this with about a half a glass of water, any carbon build up will be shocked by the temp change from the water (engine hot carbon build up and tap water) caused the carbon to break off just about everything. Afterwords go run the car hard to clean it all out.