Sync the carbs for real. It's not hard.

During that process, adjust the mixture.

Make sure that the choke plates both close fully, and that when closed, whatever triggers the throttle plates to crack open for an increased idle, is working.

Make sure your coil wire (the big diameter one) from the coil to the center of the dizzy cap is a solid core wire. If it's carbon core, it'll have too much resistance for easy cold starts. You can make your own by purchasing bulk solid core wire, off the roll, at NAPA, and they'll sell the metal ends, and rubber boots. 10min job.

Make sure you don't have too much resistance in the spark plug wires, spark plug boots, and spark plugs. Too much resistance = hard cold starts.

If you're running anything other than a standard Bosch or NGK spark plug, especially a Bosch Platinum, replace the plugs, and make sure the plug gaps are only .028". Wiz-bang spark plugs cause hard cold starting. Wide spark plug gaps cause hard cold starting.

Once this is all sorted, try starting it by pumping the accelerator pedal a few times, in order to blast some fuel into the intake. It's a big intake. Then crank her over and it should light right up.

That stumble you're getting is a lean stumble. You're cracking open the throttle plates, which causes a huge dump of air, and the carburetors aren't able to supply fuel that fast. Adjusting the mixture, and making sure the accelerator pumps are spraying into a location that creates a good spatter of fuel that the air will blow through, will make this go away.

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