Yep

you mention the tank is "pressurised". You should have a vent to atmosphere and a line to the carb - are you trying to use exhaust pressure to feed the fuel?

If the tank vent is clear, I would consider the fuel diaphragm this is the most likely place to start- possibly the needle height in the carb too.

Check for leaks around the carb mounting and crankcase - these could appear when the engine warms up - use a good sealing compound.

Start again with the tuning - start at 2 turns out on each needle and adjust in small increments