How is it cruising out on the road?
Sometimes the idle circuits will clog up but the mains are fine.
So you could have 12" vacuum at idle due to the dirt, but cruise out on the road at 18" which is about normal.
That would say dirt is getting in and messing up the idle circuits first. Usually a good clean & air blow (gasoline, then thinner or acetone, then air blow) will solve this and the carb will run good again - until more dirt gets in there.
Of course, a vacuum leak could have the same symptoms- rough idle, fine cruising. I have even had these where it will idle good COLD, but when it gets hot, the exhaust manifold expands and pushes the intake away, creating a leak and rough idle. Pesky problem. The little Fords used an integral intake to avoid this problem, which works great until you want to add a 4bbl or multiple carbs.
