To answer the question, your additional carburetor is causing the reduced vacuum reading at idle. It's the size of the venturi that creates the vacuum, and since you have two of them instead of one, it will make it easier for the air to get into the manifold where vacuum is measured.


Kevin
1951 Chevy 3100 work truck
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1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car)
Busting rust since the mid-60's
If you're smart enough to take it apart, you darn well better be smart enough to put it back together.