I got a Murray mower with a Briggs and Stratton 500e 5 HP engine from a friend. Not sure how long it may have been sitting, but I can start it first or second pull. After it starts, it runs slow, unless I tilt it back until the handle is level with the ground. When it's tilted back it runs like normal, but then it cuts off because it stops getting fuel after a few seconds. I've already cleaned the carb, replaced the gas, checked the fuel filter, checked spring tension on the throttle/choke rod (no actual choke, just a spring attached to a rod to hold the door in the carb open) and that is fine, and I checked the air filter/cleaner "sponge" and that looks clean and the engine runs the same with or without it. Only thing I haven't checked was the spark plug, but I don't think that would be the issue because it runs fine while it's tilted. Does anyone have any idea? Or should I just rebuild the entire engine?