Your basic problem...(drum roll)...ALTITUDE!

ALTITUDE ALTITUDE ALTITUDE!

Missoula MT is at elevation 3200 feet. Your engine was running rich. That's why the plugs are black.

On the carb you are using now, if it is a manual choke carb it is definitely too rich, those are built rich. If it's an electric choke carb it is built lean for economy at sea level, it may be that at 3200 ft it'll be pretty close.

On Holleys I have found one jet size smaller per 1000 ft elevation increase to be about right, for most of their legacy carbs.

I haven't had to work on AFB/AVS carbs so don't know, but Edelbrock is very good about tuning, they have put a lot of information out there.

Start with new plugs and see what you get.

R.