Vacuum Advance sticking?

Are you confident it's plugged onto the right port? There are a few nipples...one is ported vaccum, that is, no vaccum present at idle, but as the throttle is openned, the vaccum is present..It's only meant to be "on" at cruise, not idle. There are usually one or two other nipples that are manifold vaccum, which has vaccum present whenever the engine is running, regardless of load or rpm. If the hose is on the right one, I'd say you have the trottle plates open too far. Possibly a vaccum leak, or low initial timing and/or poor idle mixture. It is common for some GM centrificals to stick, but because the mopars' is way down in the "well" of the dist, I'd say it's more likely your throttle is open to the point it's uncovering the transfer slot and vaccum advance passage over the throttle plate. Unplugging it removes the source of vaccum, and the timing goes back to base, the centrifical is closed properly at that point. Slow down the idle (back off the speed screw) until you cant feel any vaccum present, then reset initial timing to 10* at least. If it cant idle, then you may have another issue.