Idle problem

I think the problem is the vacuum booster for the power brakes. It pulls vacuum from cylinder #7 via the port on the intake runner. I clamped the booster hose off and the misfire was gone, and the idle speed went down.

Trying to find out if the boosters for a 70 Duster with front disc brakes is the same as my 69 Dart with 10" front drum brakes as I have another booster here at the house.

Thanks for the help, guys. Appreciated.

George