Sounds like fuel starvation. Was the mower stored over winter with a tank of gas that has since evaporated?
I would disassemble and clean the carb and fuel tank.

BM: 1st crack a Cold One , next fill fuel tank with fresh fuel and 2 ounces of concentrated fuel cleaner . Let the New fuel and cleaner sit for 30 minutes then start the mower as usual and keep it running with the choke if req'd until the cleaner begins to reduce or resolve your fuel restriction. As RB advised if you have stored the unit for any period of time without proper draining and shutdown preparation then most likely you have fouled fuel curcuits within the carb . You shopuld also routinely inspect and clean fuel & air filter and replace the spark plug probably every second yr . Let us know how you make out . Hopefully the fuel treatment will clean out your gumming or varnish other wise you may have to dissasemble and clean the carb internal fuel circuits , unless you experienced you should probably let a knowledgeable person do this work .

I assume that this is a model w/ the carb on the tank? If so, buy a new diaphragm kit. Pull the tank and carb together, clean up the tank best as possible and install the diaphragm. Watch that the gasket goes on the carb side.