50cc AJH Athena.

Moped brains out there, all summer I have a problem with a 70CC AJH Athena.
Done a brand new engine build, but from the word go, the bike would not run, just couldn't figure out whats is wrong, still stumped.
messed with the timing... messed with the carb. swapped the carb, swapped the ignition.
compression good, crankcase pressure good. changed the reeds etc.
swapped the exhaust out, for a different one.
Still the same problem, the engine would start for about 8 seconds or so, then stall out, like the exhaust was blocked.
the crankcase and exhaust were very wet with some fuel in it.
this happened over and over again, no matter what adjustments or changes I did.
Even bought a new 70cc AJH cylinder, still got the same problem, changed the heads... still same problem. its like some timing/airflow problem.
So im thinking going have to pull the engine out and start from scratch, but before I did that, I just threw on a different kit, which happened to be 13-year-old 50cc AJH version, it looks exactly the same as its big 70cc brother, just a 50cc piston.
the bike ran really good of the bat, see the video.
so what's the deal guys why is my AJH 70CC kits not working?
yet the 50cc version does.
All i can think of is the unstuffed crank im using, is giving to much crankcase volume for the 70cc kit, messing up the airflow?