I have the argo in the garage. I took off the air cleaner and removed the carburetor. Unscrewed the fuel solenoid and used a flat head screw driver to pull out the main jet, per the service manual. Its about the size of a small screw, made of brass. It is stamped 104 on the top by the flat head screw slot. Isn't 104 even smaller than the sizes you were suggesting, meaning it should run even better at very high altitude?