To begin, I don’t have first-hand experience with this engine. And there are people on this board that have WAY more experience than I do. That said…..

I had *identical* issues with a 1954 15HP that I rebuilt. Like you, I had gas, good solid spark, air…but only got the occasional ‘burp’ out of the engine. Rebuilt the carb twice with the same results.

I’ll spare you my frustrations & cut to the chase: One day…and I don’t know what prompted me to do it….but that sintered metal gas filter that hangs under the carb? I put one end in my mouth, my thumb on the other end & blew. Nothing. Nada. Zip. Plugged solid! Took out the filter & put the bowl back on just to see what would happen: started on the very first pull of the rope.

Does your carb have a similar filter in a bowl under the carb? Might be the culprit….

And irrespective of whatever the problem turns out to be, I would add: even if you are getting spark, if the coils are original, change them anyway.