Assuming its fuel as you say the spark is good
How long after starting does it get in trouble? 1 minute? 20 minutes?
During that window of time when it is running, can you load the engine, or will it stall?
My Jube did this very badly and it was a clogged filter in the tank, as others have said.
Does it do it on first starting in the morning, or do you have to run it a few hours and then it acts up?
did anything change such that the fuel line might be now in contact with the manifold and causing boiling?