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several items.
ICM, knock sensor, some others.
BUT the ICM (Ignition Control Module) must have data about the manifold vacuum to do its job.
That data comes via a white (dirty white) nylon tube that looks like a fat wire. Look at a lower corner of the ICM for a vacuum thingie where the whitish nylon tube attaches.
The tube runs uner the front of the engine alongside the wiring harness, and comes up behind the distributor. Then it goes up to a right angled rubber elbow between intake rail #1 and #2. That elbow connects to a brass ferrule in the intake manifold. (The ferrule is just like the one for the small tube to the flame trap.) Check that the ferrule is not blocked.
If that nylon line is open, it makes a small vacuum leak and no spark advance. If it's blocked, no spark advance.
You might try this: Disconnect the line from the manifold. Let the engine idle, and (wipe it clean first) put your mouth on the line and suck, and listen for engine speed change. Not too scientific, but that's where I'd start. It's quick and cheap and revealing.
Taking the tube off at both ends and blowing through it runs a minor risk of messing up the point where it connects to the ICM, and also you can't tell if there is a leak en route to the ICM.
Good Luck,
Bob
:>)
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