My intake manifold that I have installed actually doesn't have that plugged bung beneath the thermostat... there is nothing there but the cast manifold. I do have two other spare intakes which have that bung... One of the intakes has a BVSV in that location, which apparently operates the vacuum advance in some sort of way... something about when the temp heats up, the advance kicks in...

Ken

1984 Toyota 4x4 Truck, 4.88's, 32x11.50's, weber 32/36, aux gas tank, Warn 9500i, SR5 guages, tilt/altimeter, Alpine stereo with dual 12's behind the seats.