[Jack21]

Some additional things to look for.

Assuming, no additional fuel is being added from NOS lines or fuel leakage from acc pump channel at float bowl/throttle body gasket (had to O ring the Holley to get rid of this).

Small carb on big engine could have primary curb idle opening primarys into the transition slot. Too much fuel, not enough air.

Carbs come out of the box worst case optimized for intended use. Running too rich is OK. Running too lean is not OK. Hydraulic roller cams pull a lot of vacuum, and the carbs have to be leaned out. Solid, lumpy, rollers may be the other way around, but you won't know until you put a vacuum gauge on the engine.

Your 625 is set up for a 300 - 350 sized engine with a mild hydraulic cam, making 250 - 350 HP.

Adjust curb idle so primary butterflys are at, or just under the transition slot. Adjust idle mixture to 1 1/2 turns both sides. Hook up a vacuum gage to the carb manifold vacuum line, and try to start engine. Chances are it won't start without some blipping.

Start opening the secondary butterflys to add more air to the mixture. You don't need much. With Holley, I replaced the slotted screw with a setscrew so I could adjust on car, engine running with a small allen wrench. As you open the secondaries, the RPM will pick up, and the engine will lean out. The Holley book has you drilling holes in the primaries also. I haven't done that.

Vacuum gauge reading. Power valve should be at least 2 points under the lowest idle vacuum. This keeps the power valve from opening with the engine idling. Demon has screw in air bleeds, and you may try using larger ones.

That carb is way too small for that engine/cam combination. Go 750 - 800. Put 625 Demon on a 302.