Just a little bit of water in your mower's gas can cause a lot of problems. Contamination of your mower's fuel system by water will, at best, cause frustrating performance problems, and it may also cause long-term damage to your mower. Knowing what to look for will help you prevent a ruined mower.
Water is denser than gasoline, so it will sink to the bottom of your mower's fuel tank, and most mowers draw fuel from the bottom of the tank where the water is sitting. This means if there's water in your fuel, it will be immediately drawn into the carburetor, and there's a good chance the supply of fuel to the mower's carburetor and spark plug won't be enough to keep the engine running well. If the contamination problem is severe, you might have trouble getting the engine to start in the first place.
Running and Stopping
It's more likely enough fuel is delivered to the spark plug to get the engine running, but once the engine is running, the subsequent introduction of water into the fuel supply may cause it to stop running within seconds. You might be able to start it again, only to have it die repeatedly each time you start it.
If there's not enough water contamination in the fuel system to prevent the engine from starting or running, there may be enough to keep it from running well. If your mower sputters, surges, doesn't accelerate when you increase the throttle or dies under load, you may be dealing with fuel that has been contaminated with water. Water will accumulate in the bottom of the fuel tank and in the bottom of the bowl in a bowl-type carburetor, but these signs are difficult to spot without disassembling your mower.
Fuel System Damage
Although you're likely to notice performance issues caused by water in your fuel long before you notice the damage it causes to your fuel system, the persistent presence of water in your mower's fuel will cause structural as well as operational problems. The dense water is likely to linger in all parts of the system, where it can cause rust, corrosion and other damage in the fuel tank, fuel lines, carburetor and spark plug.