Most problems that will cause a John Deere lawn mower to stall after 20 minutes occur due to the distribution and rate of flow in the fuel and air systems. If these systems are dirty or restricted, the mower will stall out and be difficult to restart. Locate the source of the fuel or air loss to get your John Deere lawn mower running again.
Improper Fuel Pressure
The entire fuel system must consistently maintain the right amount of pressure for the fuel to flow from the tank into the carburetor over to the cylinder and back to the tank. This pressure originates from the pumping of the crankshaft; however, as the fuel heats up, more pressure is generated. If the pressure in the tank can't vent, the engine will stall out, as the fuel can no longer flow in the right direction. Clean the fuel tank cap and make sure the vent hole isn't plugged.
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Blocked Fuel Passages
Dirt, grass or other foreign matter can drop into the tank accidentally during a refill stop. As the mower starts sucking more gas into the carburetor, the matter eventually gets sucked up against the fuel filter or into the fuel lines, causing a drop in gas, and the engine dies. Drain all of the fuel remaining in the mower's tank and look for dirt and other material. Scrub out the tank with a brush and rag if it's dirty. Take the fuel filter off the end of the main gas line and replace it.
Clogged Air Passages
The engine sucks air in to mix it with fuel for combustion. This incoming air also keeps the engine cool, venting off some of the heated gases. The rest of the heated gases fly out of the muffler and away from the engine. If these two passages get clogged or blocked from dirt or carbon buildup, the engine will start and stall out soon after it heats up. Clean the air filter after every eight hours of use and scrub out the muffler every 60 hours.
Carburetor Settings Off
If the jets on the carburetor can't pull in enough gas for the engine's speed, the engine will shut off automatically. Three screws control the settings of these jets at the idle, low and high speed. If any of these screws, especially the idle and the low speed screws, are improperly set, the engine will stall. Adjust these screws only if trained and instructed to do so. Minor adjustments go a long way, so if you're unfamiliar with these procedure, take the mower to a mechanic.