Mystified by push power mower. No gas?

Poulan (Electrolux) 4 hp recoil start. Model 96112002500, catalog PO45N22SH. Many years of faithful service, it still has good compression. I went to start it and it only keeps running as long as I keep spraying starting fluid. I can see the bulb sprays gas into the throat. Took carb off, blew it out, checked for cracks and loose fitments. Carb passageways seem to conduct air when I blow through them. No needle valves, the carb sucks gas directly from the tank. To me it looks like this engine should start. Left old gas all winter, so now I'm paying the price. How to de-gum the carb?