My son in law ran out of gas and mower wouldn't start back up. Skip all the usual troubleshooting, finally replaced the carb. Now the only way the mower will run is holding hand over the intake to choke it. As soon as you allow a little air by moving a finger just a tiny amount, the engine dies. I've got good fuel flow to the carb, good fuel flow in the bowl of the carb. I'm lost now.