Can't read vacuum for 66 Dart

I'm trying to read the vacuum on my Carter 1 barrel on a 225 but get nothing. I put the gauge to the distributor port since that's the only spot on the Carter. I want to know if I can't do it on a Carter or if something might be up with the carb because the idle is kinda rough too but that could be the old points. Anyway does anybody know what I should do to read the vacuum? Thanks