You don't have to swap the vac adv unit until you know for sure it's bad.

Shoot the timing with vac adv disconnected, note the amount of advance.
connect the vac adv and shoot the timing again to see if it's holding at where the manual says it should.

Another test is to put the vac advance hose in your mouth, suck on it, see if the timing plate moves.
While suction is applied put your tongue over the hose end and see if vacuum remains or bleeds off.
Kinda crude, but it works.
(Engine off when you do this one.)


Any chance you have a reverse flow pump?

Got a shroud?

Is the engine bay stock with all the air blocking devices in place so air will go through the radiator?
Air takes the path of least resistance and will go around the radiator if it can.