IF it came through my gate then did what you said the air test is the first thing I do which saves me pulling the carb off the tank and replacing a perfectly good diaphragm & gasket.
I have assumed you fitted them correctly, diaphragm to the tank.
The induction tube ( manifold if you like ) is a thermo setting plastic so will shrink & crack with exposure to sufficient heat .
Also doubt the security of the O ring seal between the carb & induction tube .
Well the diaphragm is placed correcty and carefully. The intake manifold on this one is metal but ive seen the plastic ones on ther ones i have fixed before.
i did blow the carb with compressed air yesterday but that didnt fix the issue.
when i add a bit of fuel thru air intake it turns on and dies ss soon as that fuel is burnt off.
i will remove the carb again and check everything but i did noticed after emptying the tank and cleaning the carb and putting it back on there was fuel in the reservoir (or shatever its called) under the carb.
i did also remove the cylinder head snd inspected the valve seals and piston and it appeared to be fine.
took the oil out and refreshed with new oil as the old oil was black as.
im suspecting the carb but its a very simple thing and pitty i cant remove the bloody fuel filter tubes to clean inside the them thoroughly. Very bad design. But shen seeing the fuel under carb would have thought its sucking he fuel therefore its working.
As oppose to the carbs with manual priming button which shoots fuel through to the manifold into the cylinder i cant figure out how these auto choke carbs actually shoot the fuel into the manifold.
there is no where i could see in it where fuel could be sprayed into the manifold.