I keep getting gas in my oil. Have a Briggs & Stratton Engine with 19.5 When carb is off engine I can put a short gas line to gas inlet and carb up right and blow into it it. Turn the carb upside down and can not blow into it. It has a Nikki Carb on it. I have cleaned the carb and have checked to make sure the fuel solenoid is working. Even took it off and checked that way. It is moving up and down when I turn key to on position. I work on small engines as a hobby and have run into a number of these. What am I not seeing?