As the lawnmower engine burns gas inside the cylinder it generates an enormous amount of heat. This heat must disperse out of the engine to keep the temperatures cool enough to maintain the proper pressures for all the moving systems on the lawnmower. When a lawnmower stalls after heating, generally too much heat or pressure has built up in the engine.
A simple, often overlooked, trigger for an engine stall occurs when the heated gases inside the fuel tank can't vent properly. The pressure causes the gas flow to reverse and move out of the carburettor, effectively shutting the engine down. Often referred to as vapour lock, the engine can't operate with these heated gases trapped inside the tank. On lawnmowers, most vapour lock occurs through a dirty fuel tank cap. These caps have a small vent hole that allows the tank to maintain the proper pressure. Clean or replace the fuel cap.
Gas Oil Problems
Problems in the supply of fuel and lubrication will cause an engine to shut down, especially as it starts heating up. Always check the fuel supply before each use. Dump out any gas older than 30 days, as it will go bad and clog up the fuel system with a sticky substance. If you used bad or old gas in the engine, the problem may require a thorough cleaning of the fuel tank, replacing the fuel filters and fuel hoses. Always check the oil supply as well and add more oil as needed. Change the oil after every 25 hours of operation.
When bad or old gas goes through the carburettor it starts leaving behind a gummy deposit. This deposit, as it builds along the hoses and inside the carburettor, will gradually choke off fuel arriving to the carburettor. If the carburettor doesn't get enough gas the engine will shut off. Problems in the carburettor, such as a stuck intake needle, will restrict the fuel. Remove the carburettor, disassemble and clean it thoroughly. Install a carb kit before reinstalling the carburettor.
When the engine can't vent the heated gasses from the internal combustion or take cool air in for the next stage of combustion, stalling can occur. Air comes in through the air filter and goes out through the muffler. The air filter gets dirty quickly, so clean it every eight hours of operation. Clean the muffler after every 60 hours of operation. Replace either the muffler or air filter if it's too dirty to clean.