If there's air coming out your exhaust, then there's air coming into the engine. Put your hand by muffler-outlet and feel for puffs.
Improperly plugged ports on carbs would cause fuel issue. Might be too much fuel? Float-bowl vents should always be opened. Vacuum-sources should be plugged if they're not used for anything. Do you have vacuum line going to petcock?
Test for spark with plug outside engine. Insert plug into wire and ground outside. Lay on metal of engine or alligator-clip to chassis. Do you see spark jump gap when you crank? Is it a strong thick blue/white spark? Or a thin orange/red spark?
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