Re: Holley 670 SA on a ZZ4 350HO won't idle down
If the cam is free to move, yes, it is not on the fast idle setting/screw.
Eighteen inches of vacuum is excellent.
If place your hand(s) over the carb air horn, the engine should stop.
I would try the secondary side first because as suggested, the secondaries appear to be open slightly.
You can place some Scrotch® tape over the secondary transition slots ensuring it doesn't leak from them, including PCV.
Have you plugged each and every vacuum hose/port from the intake and carb?
Tighten the intake bolts?
You mentioned normal dual plane manifold. Holley is square bore, unless you have a Model 4360, 4165, or 4175 Spread-bore carb, matching a spreadbore manifold, but with 18" of vacuum, it seems to be covered.
As a last resort, change to a known carb and/or, turn down the mixture screws to the rpm desired.
Are you certain the carb does not have reverse-mixture metering plates? Very popular with emission carbs of the '70's and not saying a plate may have made it into the production mix, but you have to eliminate the variable.
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