after assembling a very sorry Yamaha KT100 2 cycle kart engine - it is not pumping fuel. I was a little surprised that the line from the engine to the crankcase actually seems to be pushing air - I was expecting suction. is that correct?
yes a two stroke builds pressure in the crankcase, totally different design than a 4 stroke. the pulses will operate the external pump if you are fitted for one
When the piston goes DOWN, it pressurizes the crankcase - that's how the air fuel mix gets from the crankcase -> up through the transfer ports -> into the combustion chamber.
When the piston goes UP, it makes a vacuum in the crankcase.
That is how the air/fuel mix gets sucked through the carb.