I am working on a B&S 6.5HP on a push mower. Replaced air filter, spark plug, gas oil, and rebuilt carb. Lawn mower will not cold start when choke valve is completely closed. I can manually open the choke valve, and the mower starts fine from a cold start. Once the mower is warmed up, it will start without having to manually open the choke valve. The mower has a ready start auto choke system. The thermostat/spring that controls the auto choke seems to move fine. In its resting state, it has the choke valve closed, which will not allow the mower to start. Any ideas?