Hey guys, thanks for the suggestions. Checked the fuel lines, all good - but by pure luck it put me onto the actual problem.
I hadn't seated the seals under the black plastic end caps properly - the little copper tube was crooked on both of them. So the slides were moving back and forth WAY too easyilty (no suction noise/resistance when i played with them). Re-seated both seals, running fine.
My knowledge of carbs is limited - if someone can chime in and tell me what effect that would actually have when running the engine, that would be great. My theory is that when I hit the gas, the slides weren't opening (no suction in the chamber behind the black end caps), and so I had bugger all flow into the cylinders.