I can't open the fast mixture screw enough to prevent the engine from stalling at full throttle.
There are two tubes exiting the gas tank. All parts diagrams seem to show the smaller tube (.140/.080) running to the carb fuel inlet, and the larger tube running to the purge bulb outlet.
Is this correct? Maybe the smaller tube can't handle the fuel flow needed at full throttle?
What is bigger,
The hole in the fuel line or the hole in the main jet ?
Add to that the fuel in the metering chamber is under pressure.
SO not the thickness of the tube is not your problem.
A blocked fuel filter in the tank, a blocked fuel filter in the carb a cracked fuel line inside the tank, a stuck check valve in the carb, a blocked jet, or a leaking manifold, leaking crank case seal.
These could all be causing the lean stall.
Are you sure the engine is stalling lean