[1] The first thing to check is that the passages in the AFB base are covered by the gasket/manifold flange. If not covered, you will have a large air leak which would explain your some/all symptoms.
[2] Engine idling, remove the PCV & make sure it is pulled in. If not 'in' or vibrating it is adding varying amounts of air & playing havoc with idle air metering.
[3] The cheap [ & simple ] fix for [2]: get a 3/8" diam bolt & cut off a 3/8" length of it. Drill a 1/8" hole through it; push into the PCV hose. You now have a functional PCV.
[4] The ProCrap 850 probably had adjustable by pass air, & this carb may need it too.
[5] Not time for drilling holes....yet.
[6] Single plane, & a Victor, will provide less vacuum & exacerbate the problem.
[7] What are the cam specs & timing at idle? See below.
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