A friend of mine that owns a GP had a similar issue.
It turned out that the fuel line clogged.
The GP has a flat fuel tank, and the fuel line is conected directly on the bottom of the fuel tank.
Any dirt will fall in the fuel line and restrict fuel flow to the filter and pump.
My advise is to disconect the fuel line at the carb, and manually pump the primer lever with the engine turned to the point full strockes are possible. (cam lobe for fuel pump down)
The fuel should squirt out of the line each strocke from the primer lever, and you should be able to pump at least 1 quarter of fuel without experience any mis or poor squirts.
If no good fuel squirt's, undo the fuel line from the bottom of the tank and also from the inlet port of the fuel filter.
Blow it through with air pressure.
Use a 3 way conector, and install a small sediment on one side of the conector, and screw the oposite side in the fuel tank.
The third hole is for the fuel line.
Modifie the fuel conector port that screws in the fuel tank with a small stand up pipe that comes about 2 inches above the bottom of the tank.
The fuel pipe is peened close above, and small holes drilled in the shanck from the stand up pipe over the totall lenght and radius.
This prevent thick dirt from clogging the lines and small dirt sinks into the sediment bowl.
It is a very rewarding mod for the GP.