Yes, that's the vacuum source to the ICU to vary timing based on load (vacuum).

I'm not an expert on either ignition or emissions, just passing along a tip I found on the net. See the links I posted for details.

This has worked for 2 people that I know of, but it will NOT compensate for a bad cat or poor state of basic engine tuning & condition.

I imagine that applying suction to the ICU would affect timing somewhat, but can't say how much or what effect you might notice.
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Bruce Young
'93 940-NA (current) — 240s (one V8) — 140s — 122s — since '63.