Your carb butterfly linkages look at slightly different angles to each other, do they all open exactly in synch when you look through the throats?
Also whats that spare wire with circle connector hanginf off your solenoid?
Is the black primer solenoid handle right, looks wrong to me.
That is correct for the " run " position.-------Where is the silencer that mounts in front of the carburetors ?----Have you confirmed that apark will jump a gap of 7/16" or more ?
The real clue to your problems is staring you in the face. The idle fuel quantity and mixture is critical to proper operation of the engine.
Your mixture screws are not all the same, and they are not in the proper range...2.5 turns. They are way off.
Correct that before you can call it fixed.
As for testing the spark, no. I'm not saying it is impossible, but I feel if it was a spark issue, I wouldn't be able to make it run correctly at all. I can get it to either idle or run on the top end. Now it just wants to not run well when hot and is sometimes hard to restart.