You don't have to leave your lawn mower out in the rain to get water in it, although that will probably do the trick. You can get water in a gas lawn mower just by leaving it in a shed all winter with a tankful of gas. Gas draws moisture from the air, and the water settles to the bottom of the tank where it's first in line to enter the carburetor when you try to start the lawn mower in spring.
A lawn mower submerged in water will get wetter than one left in a shed all winter, but either way, you've got to get the water out if you want the engine to run because — and you probably know this — water doesn't burn. Water also corrodes the metal parts of the mower and can cause permanent damage that ends the life of the mower. The cleaning job starts with the fuel tank.
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Getting Water Out of the Fuel Tank
You won't find a suggestion here to turn over your lawn mower to empty the fuel tank; that's messy and probably won't work. You could siphon out the fuel, but it's better to remove the fuel tank. Start by removing the spark plug and fuel filter and then find the screws holding the tank to the motor housing and unscrew them with a socket wrench. Wiggle the tank a little to unseat it and when it's free, lift it a little and pull off the fuel line. You have to replace the fuel filter on one end of the fuel line, so remove that, and it's not a bad idea to replace the fuel line, so pull off the other end from the carburetor housing.
Empty the contents of the tank into a fuel-safe container and take it to a service center for disposal. Wipe down the inside of the tank with a towel wrapped around a stick and then spray compressed air or some WD-40 into it. "WD" stands for "water displacement," so this is actually a correct use of this product. Let the tank air-dry while you continue the cleanup.
Cleaning Water From the Oil Reservoir
Most lawn mowers these days have four-stroke engines, which means they have a separate oil reservoir (two-stroke engines use a mixture of gas and oil). If water has gotten into the oil, the oil will turn a milky white color, and you'll be able to see this in the end of the spark plug. That's bad because it means the oil isn't lubricating properly.
To get out the contaminated oil, you will have to tilt the mower, so move it onto a level surface and put some newspaper underneath it. Remove the dipstick, place a container next to the oil port, and tilt the mower toward the container to allow oil to flow into it. Gradually increase the tilt until the oil stops flowing and then dispose of the contaminated oil along with the contaminated fuel.
Cleaning Water From the Carburetor
If the water got into the engine by being absorbed by the gasoline, some of it has probably gotten into the carburetor, and when that happens, it often leaves gooey deposits that stop the flow of fuel. You can clean the carburetor with carburetor cleaning fluid, but to do that, you have to remove and disassemble it. Use your owner's manual to guide you for this procedure. If you can't clean the carburetor thoroughly, you may have to replace some parts or all of it.
After cleaning everything, reassemble the parts you took apart; put in fresh oil and fresh gas; install a new spark plug, fuel line, and fuel filter; and start the engine. It may sputter at first, but the heat generated by the engine should take care of any residual moisture you missed, and after a while, the engine should run smoothly.