Lawnboy mower starts but won't stay running

The Mower needs to have the Carb Cleaned and Rebuilt. The Main Fuel Jet is Starved for Gas. This is a Common Problem at the Start of every Season. Always Store the Mower with the Carb empty of Gas. Shutoff the Gas Flow from the Tank and Run until Engine Quits. This will Prevent the Problem you are having for 3 or 4 years. Now it is Ready for the Winter Season. Hope this Helps.

Good Luck

Hi,
Always best to say what engine you have; Lawnboy still uses two brands since the EPA "killed" their engine.
Previous answer is correct; though how to solve it most easily cannot be answered without knowing the engine.
Peace,
Clarke