yes sounds like a leak.....
Okay you sealed the manifold joint
you sealed the manifold gasket
you tightened the carb screws
so where else could it be leaking...
Head gasket
butt gasket
have you ever removed the head or even loosened it.
well beats me but it seems the only place left for air to infiltrate. BGJ thinks so as well. I think I would lift the jug seal it with sealer and try it again. Unless norman tells you not to use sealer. Seal on both sides of the gasket. You can probably do it without removing the jug. just lift it a little and do the seals.
It's getting very hard to follow this thread so please excuse me if this is of no help:
To test for air leakage , if motor dose not stall with choke ON, you have an air leak.
When leaking, you tend to run LEAN, when you put the choke ON, you are enriching the mixture with more gas.
Yes norman has a point but when I did mine I went with the oring and no sealant.