No start 340

Im not sure how to diagnose sputters , however I would look at what has changed since the dyno.
Fuel , is the fuel fresh , does the carb have a stuck needle and seat
Ignition , what has changed , plugs, gap , coil , MSD box
You can move the distributor 180 degrees and it does not backfire? confused by that.
You have a good response above and if possible post a video, good luck.