To stop your riding mower from vapor locking ensure that the fuel lines are located away from the hot engine components such as the exhaust manifold and use heat shields around the fuel lines and carburetor.
And check to ensure that your fuel cap is allowing air to vent and use fresh and good fuel.
You can also add a fuel line insulator to prevent heat transfer from the engine to the fuel line.
Fuel lines that are routed close to the engine and exhaust can cause the riding mower to vapor lock and stall out.
Wrapping the fuel line in some heat resistant insulation can help prevent the fuel lines from getting too hot and causing vapor lock.
Other reasons for a lawn mower including riding mowers and push mowers to stall or die out can be due to cutting too thick or tall of grass and overloading the engine or it also could be a result of a clogged fuel filter, clogged main jet in carburetor, fouled spark plug, dirty air filter, bad coil, loose or damaged spark plug wire or even low oil.
The lawn mower may also be flooding itself out due to a needle sticking open in the carburetor and allowing fuel to flow too fast through the carburetor.
The carburetor may also need to be cleaned or replaced if it continues shutting off or the ground wire that is designed to kill the engine may be loose and grounding itself out to shut it down.
A bad spark plug can cause a mower to stall.
The spark plug is responsible for igniting the fuel and air mixture in the mowers engine and if the spark plug is not sparking correctly then it won't ignite the fuel and air.
The bad spark plug can cause the engine to stall on the mower and even misfire or not start at all.
You can take the spark plug out and place it against the metal part of the engine to ground it and then pull the rope on the push mower or if on a riding mower crank the engine and watch for spark.
If the spark looks weak then the spark plug may be bad.
It could also be a bad coil as well causing weak spark.
A fouled or pitted spark plug can create an inconsistent spark and lead to uneven engine power and stalling or hard starting of the engine.
Other things that can cause a mower to stall are clogged fuel filter, bad fuel, water in fuel, overloading the engine such as cutting through tall grass and weeds.
Spark plugs for mowers are cheap and easy to replace.
Due to carbon buildup and a weakened electrode, the spark plug will degrade over time which can cause the engine to stall.
You should inspect the spark plug for damage or wear and you can also use an ignition tester to help determine if the spark plug is defective.