2010 20 HP Merc Won't Run at Low Speeds without Choke

I have to push my throttle only button and goose the throttle to about 1/4 to start the engine. It then will idle down, still with the choke out, so I can twist the gear shifter and quickly rev up above half throttle to keep the motor from dying. Only then can I push in the choke without killing the engine (or it goes in by itself), and if I let up below 1/2 throttle, the engine dies, having to be started with full choke and a goosed throttle again. This is a brand new motor, installed with a new gas line, new Racor water seperator and new gas tank, with fresh gas. The dealer says the carb is icing up. I say, bull. This is Alaska and Mercs run here in all conditions. He said, drain the carb. I did. Same, Same. Any suggestions? The motor is too new (1hour) to just unbolt and let sink. Thanks for your help.