5hp quantum not staring

New to the site and have a few question. Bought a MTD 5 hp quantum self propell from someone who needed to some money and didn't even know if there were any issue. Don't really need the mower but wanted to check it out. Tried priming the engine and couldn't get it to crank. Wanted to see if it was firing, removed the spark plug and put some gas in the plug hole. Mower would crank and run until it ran out what was in the head. Pulled the carb and replaced hoses to the primer and cleaned the carb. My question is about the primer, usually the primer is on the carb but this one is under the tank. When pushing the primer, it only pushes air into the carb. Not sure how it suppose to work. Still haven't gotten it to crank, and still need to put a kit in the carb. Can someone explain how the primer work on this motor. Failed to mention it is a Briggs and Stratton 5 hp I/C engine

Thanks