So if it isn't the carbs or some other fuel issue (that it really sounds like but has been worked over) then I'm back to pulling straws out of my butt.
This sticky valve business.... the engine runs on both cylinders for a few seconds before one stops and this makes me wonder if a valve is not getting warm and expanding in the guide, causing it to stick open temporarily. The cam will have the power to open it, but the spring is too weak to close it all the way again. The problem could be either a weak spring or a tight valve guide. And this is all just a wild guess on my part.......
