Matt...

It's VERY common for the brass nipple in the manifold AND the small vac line to plug solid with carbonized varnish.

If you unplug the small vac line at the flame trap, you should feel vacuum at idle. If not, check the hose and the nipple for blockage.

Yes, when you had the manifold off you should have replaced or cleaned the oil separator. With the manifold on, it's about as tough to access as the blower motor.

However, the oil separator doesn't plug as easily or often as the flame arrester or the small nipple. You can assess the oil separator by simply pulling off the top hose and allowing the engine to idle. Every engine has some blow-by, so you should see vapors puffing up from the crankcase. If no vapors, then the separator's likely to be plugged.

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Don Foster (near Cape Cod, MA)