You know for a fact your getting good spark at the plugs?
On last rebuild I did, lol, new everything all around. On start, pure ran like crap! Chased things down for two days, re-checking everything half a dozen times. Wound up...only 3 plugs, out of a new set of 8, were firing right. Another new set of plugs, fired right off.
I agree with the guys...sounds like 180 out on timing. If things were firing right, you would be getting big time back-firing.
Real easy to get confused on TDC compression stroke, depending on the cam. Valve overlap, and that sort of thing. Sometimes, the finger on the spark plug hole doesn't cut it.
Simple thing (if nothing else), to reset dizzy 180, and see what happens. If it's firing off...it will let you know...one way, or the other!!
Just for the heck of it...TDC #1...both valves will be closed, either stroke. But, on exhaust stroke, they will be overlapping the amount the cam allows.