How can you tell if the vacuum advance is not working properly? The vacuum pot on the distributor has two connections, one is used and the other one is plugged. The one hooked up that runs to the carb is connected o the timing retard side and the advance side is plugged. The engine is a 1970 302 from either a Bronco or 4 wheel drive truck.

 

Well, that is surely not right. I'd be inclined to replace that vac can with an aftermarket non-smog piece, and connect it to manifold vacuum. The advance/retard cans usually didn't have as high a range of movement as the non-smog types. Accel / Mr. Gasket sell adjustable cans pretty cheaply.

 

Terry those dual port distributors were designed for the original stock application. I've seen people use just the advance side. You really need to check your advance curve for the way you are setup now.

Like Ross said, you can probably get an after market vacuum can and dial in your mechanical and vacuum advance for your current application.

I have even bought reman stock units for Mustangs or other vehicles with the proper advance curve. O'Reilly's or Autozone have them and they were pretty cheap when I bought them a few years back.