After a lawn mower starts running, the normal combustion process allows it continue running for as long as a supply of fuel, air and spark is available. When the lawn mower sputters, or idles roughly, then quickly stalls, this problem usually relates to either a loss of air or fuel. A thorough cleaning and inspection of the air and fuel systems will solve most minor problems with these symptoms.
When the lawn mower starts combusting and heating up, it needs to pull in a greater supply of air, while also moving the increased amount of burnt gasses out of the cylinder. If this air cycle gets interrupted or blocked, the engine will continue burning fuel, but poorly, and soon the engine will get too much fuel and shut off. For this reason, keep the air systems clean, so the air can always circulate into and out of the engine. Wash the foam element inside the air filter box with soapy water and replace it ideally every season. Clean out the spark arrestor screen and muffler with a brush after 60 hours.
Checking the Fuel
Usually though, this problem occurs due to a decrease in the supply of fuel reaching the mixing chamber on the carburetor. When the fuel supply drops suddenly, the engine will start sputtering and eventually stall when no fuel reaches the carburetor. Unfortunately, this drop in fuel can occur at numerous points in the fuel cycle. Outside of completely disassembling the fuel system, check the fuel tank for debris, clean the fuel filter and, if necessary, replace both fuel hoses.
Adjusting the Combination
With a clean air system and a fresh, consistent supply of fuel it will be safe to adjust the carburetor. Most lawn mower carburetors allow the operator to manually adjust the combination of fuel and air reaching the carburetor at the idle, low and high speeds of the engine. By adjusting these screws, the operator can increase the amount of fuel reaching the carburetor, which will help ease the sputtering and keep the engine from stalling.
Consulting a Professional
Sometimes the problem goes beyond a dirty air or fuel system and exists deeper inside these systems. Generally if these problems don't go away after these systems are cleaned, the carburetor is in need of a cleaning and repair. In these circumstances, consult a professional service technician to clean and fix the carburetor for you. Major engine damage can result if the carburetor gets adjusted poorly or assembled wrongly. Don't attempt to adjust or disassemble the carburetor if you aren't comfortable making these repairs.