Gareth,

What makes you think so?

It sounds to me as the low pressure at the Venturi is diminished by air leaks thru other points of the crankcase.

In nose up attitude, that marginal low pressure at the Venturi cannot pull enough fuel from the spray bar.
May be the muffler pressure that the tank feels is not high enough; do you have the factory diaphragm in it?

I would cover the carb and sumerge the body in water, then blow thru a hose connected to the exhaust.
Most os the air leaks are reparable, except the ones thru the flat bushing-bearing.

Just a quick test before quitting on the machine.