Allow me to try to get us back on track again
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Most likely is the power valve at this point.
I wouldn't say this is "most likely", even though we all know that the Holley PVs are lousy quality. But Adrian writes it's a new carb. So PV should not be top of the list. Besides, a hardened power valve usually stays open instead of being pulled closed by vacuum, so it's more likely to create too rich than too lean. And therefore unlikely to create backfire in the carb.
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If you are using an automatic choke, make sure that the choke is set right in the middle of the dial on "index" and that it is working correctly.
If you try to start the engine by blocking the choke open it will raise the possibility of the engine backfiring, particularly if the air temp is cold.
If it does backfire it is probably from that, i.e., the choke making the starting mixture too lean and depending on the timing of the cam events, make it backfire just a little, or a lot.
Adrian hasn't named the choke, maybe it's electric, maybe it's manual, maybe it's missing. But of course, if hard to start when cold, proper choke action should be checked
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You can confirm a bad power valve by carefully draining the fuel from the carb through the lower fuel bowl screw holes, then removing the primary fuel bowl and metering body.
If there is evidence of fuel in the vacuum chamber, if it is wet, between the metering block body and the main body of the carb, then it is leaking through the power valve diaphram.
That is a blown power valve.
The simple test when you get it out is as follows:
Create vacuum with your mouth on it, close the hole with your tongue, see if it stays there or seeps
If that's OK, test if it's going hard by pushing it gently against a new PV of same value, they should move in a synchronized way and with no "jumps"
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I think you should have around 14-15 inches with that cam
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The best vacuum at idle with that cam is going to be in the 12-14 inch area
Which is it?
Neither actually, it's very difficult to tell what the max vacuum can be based on engine configuration
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Fuel with even just 10% ethanol will cause the float to be set too high
What? Never heard that before, defies physics I think. Regardless, set the floats, has to be done on any new carb because the fuel pressure varies in every car
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Once you have confirmed that the power valve is good, I would recommend that you use a spray can of engine "starter fluid" sprayed into the carb.
I would not recommend that. Using starter fluid is just to mask a problem, so only use it if you've tried everything else to start it
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In my cars that tend to sit for too long to retain fuel in the carbs (which is most of them) I've installed and electric fuel pump IN ADDITION to the mechanical pump.
Rocky too I see. Is that added complexity worth it? Yes it takes 10 seconds on the starter to start mine if it's been sitting long, annoying. But on the other hand it allows the oil in the engine to get to every cavity before it fires, maybe that's not so bad
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So back to what Adrian actually asked about. A cold start problem and a low vacuum problem on a recently rebuilt engine with a new carb. I suggest fixing the vacuum issue first, that is probably only due to the ignition and carb not adjusted right. When that's fixed it'll probably start better. Had a friend's friend's Mustang by, ran poorly, very hard to start. Timing was way off, set it right, both problems gone.
If it still starts fine hot but not cold, work on the choke first.
Just my $0.02
