It may be something simple but could be severe. Pull those plugs back out, check the gaps, any sign of water or white insulators stop there, if all plugs look normal you probably just need to replace the wires and check the firing order. You said some work was done on the carb so lets pull it back off and check the gaskets and such. Who rebuilt the carb. The Quadragunks can be complicated
judging by the fact that u said it backfired on the one startup, it would probably be a good idea to real quick check your carb, and intake manifold gaskets. Just by spraying carb cleaner at the gasket all the way around while its running. If theres a leak the carb cleaner will find it and momentarily the engine will run smooth. Id ssy if you check all your basics, and do this your on the right track. Good luck.