Hi Blumby, at full throttle the black piece should only be about a quarter of the way off fully closed(so nowhere near half way). There isn't much difference between idle and full throttle on these setups so the lawnkeeper is about as good as its gonna get without going to too much trouble. As for the vantage judging by the symptoms(fully revving out then dying) try a couple of simple checks first; Is the carby sealed on the manifold correctly? Remove the blower housing and check if the manifold is loose or cracked.
Swap the 2 fueltank/carby setups around (as complete units) to see if you get the same symptoms or to at least eliminate the possibility of a dodgy carby diaphragm.
