First try to start 2.0 carbs
Tries to start but white-blue smoke from cyl 3 through carb with POP. immediately shut it off before it started.

Valve adjustment gone wrong on that cyl? Carb setting wrong (static setting via Dellorto book) ? Timing not moved since it was a running motor.

What's the order of things to check?

Are you missing any hardware that might be holding the intake valve open on #3? A lean miss fire will have you popping up the carb also. What carb set up?

Plug wires in the right place?

--DD

Dizzy 180 out, or your plug wires are 180 off. Give it enough fuel and you will blow pretty wild fireballs out the top of your carbs. BTDT.

Zach

If you didn't move the dizzy that's not the issue. Wires on in the wrong sequence, valve adjustment, burnt valve, sticking valve, valve seat failing.