I'm not experienced with these motors but it sounds like the diaphragm
in the fuel pump is torn, I think you should be able to draw a light vacuum
on the hose going in to the fuel pump from the crankcase and it should
hold. That pump uses the +/- pressure in the crankcase to operate and
when the diaphragm tears it lets crankcase pressure blow out through
a vent hole on the pump housing. Hope this helps!!