On a freshly restored tri-power I had a a similar problem after opening the outboard carburetors.
I had to add a heavier duty torsion spring (from a 4bbl) on the rear carb so that the outboard carburetor butterflies would close completely. You need sufficient spring pressure to return all the butterflies to idle.
Mike told me I can tweak the lighter duty spring to get some more strength out of it too.
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1965 Pontiac GTO
455/469 w/ #48 Heads, '65 Tri-Power
9.25:1 CR
Stump Puller Cam
Muncie M22W 1st-2.56 2nd-1.75 3rd-1.37 4th-1.00
3.55 Rear Differential
Front: 225/60R15 Height: 25.6"
Rear: 275/60R15 Height: 28"
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