This is now getting more than a tad frustrating. Run it with shut off valve off for over 30 mins, even in gear with a little throttle. Looking at how the fuel lines are connected I cannot see how the shut off valve could affect the flow, in the exploded diagram attached the integral fuel tank is connected to 33, there is a hose connecting the crankcase to 9. The hose connector adjacent to 9 has no hose. The fuel pump has a hose leading from the external tank connector and also a connector with no hose.
This arrangement makes no sense to me, I’m beginning to think that the previous owner had done some work on it and got a bit lost when putting it back together. Unfortunately I’ve not been unable to find a definitive schematic of the correct fuel line arrangements. Hopefully someone here will know what it should look like.