Alan, just out of curiosity are you hand flipping the prop. If I remember correctly the prop is a gear [belt] reduction drive. Have you tried using an electric starter? The magnet in the flywheel has to spin past the pick-up at a good rate of speed to generate a spark.This is why most Q42's have a spring starter on them. Also how are the fuel lines and the fuel tank? Just a couple of thoughts. As you say it will run on the prime, it still sounds like a fuel supply/carb problem. I have had problems in the past with the diaphragm that operates the needle, it becomes inflexible to the point that it won't push down on the needle operating arm, not allowing enough fuel in. Also check that the steel cover plate[ on the diaphragm side]has an unobstucted air hole. If I can think of anything else I will let you know.
Cheers, Dave.
P.S. Have you tried opening the needles some more? If you have to go more than 1-3/4 to 2 turns there is a blockage, or not enough fuel getting in.