Sounds like a partially clogged main jet in your carburetor.
This will require taking the carburetor off your lawn mowers engine, disassembling it and then soaking it in some b 12 chemtool parts cleaner.
After you dip the carb you then blow the jet and other holes out with an air hose.
Then rinse the carb off and reassemble it.
It's recommend to get new seals for it and do a complete rebuild of if you want you can buy a new carb for it.
If you clean it right it should run like a new engine.