Every once in a while everyone makes a mistake; including rushing inside the house only to realize hours later that the lawn mower was left outside, unprotected, in the rain. Troubleshooting a lawn mower that won't work because it got wet isn't as difficult as you might think.
Purchase a can of starter fluid from your local lawn and garden center.
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Remove the air cleaner, turn off or cover the choke, and spray starter fluid down the carburetor.
Turn the ignition on and attempt to start the lawn mower. If the mower starts, turn it off, remove the air filter, clean it, and set it out to dry. Put the mower back together once the filter is dry. If the lawn mower doesn't start, continue to the next step.
Drain the gas and oil from the lawn mower. Remember to drain the gas from the carburetor as well. You are removing these fluids because it is possible that that water got into these compartments.
Replace the gas and oil with fresh fluids. Attempt to start the lawn mower. Do not read any further than this if the lawn mower starts.
Replace the spark plug in your lawn mower.
Attempt to restart the lawn mower after inserting the new spark plug. Do not read any further if your lawn mower begins working properly again.
Turn the fuel shut off valve to the off position and then check to see if the coil inside the lawn mower is rusted.
Remove the coil and clean it with sandpaper or a small wire brush.
Clean the magnets on the flywheel inside the lawn mower, replace both the flywheel and coil, and tighten the coil to its original position. Restart your lawn mower