I think that one has a Pulsa-Prime carburetor. The carburetor's fuel pump generates suction on the feed from the long pick-up tube that reaches down into the tank. The primer picks up fuel through that same tube. If the pick-up screen, or the tube that follows it, is blocked, there will be suction in the primer and no fuel feed to the fuel well, which is where the main jet picks up fuel. My guess would be that you have a "floating mass" somewhere in the primary system, consisting of the tank, long pickup tube, fuel pump and primer. After it has been standing, initially the primer works and the fuel pump works, delivering fuel to the fuel well. However at running speed the fuel pump picks up vastly more fuel than when you are priming it, pumps it into the fuel well, and lets it overflow back into the tank. Your floating mass may then be moved by the high rate of fuel flow, blocking either the screen on the bottom of the long pickup tube, or the tube itself with the associated drilling that communicates with the fuel pump.
So, maybe you've missed a floater in the fuel tank, or in the pickup system?
