Matt, How about trying a different oil in the fuel? I know you have a puzzled look on your face and are saying to yourself that it's not drinking time in CA yet however, What I am thinking is that it works great at home and maybe it's being stored in a temp controled enviroment. When you get out to the field where it is colder the oil from the fuel that has now evaporated leaving just the oil and is cold so the viscocity is higher just may be enough to keep the reeds closed under crankcase vacuum. As you prop and prop the new fuel slowly works it's way in and unsticks the reeds. Far fetched but maybe worth a try?