If you have a gas lawn mower, every time you use it, before storing it, make sure the mower runs out of gas. You'll stop the carburetor from gumming up. Also, they make a fuel additive for lawn mowers, that helps clean the carburetor. This will make the mower easier to start the next time. Remember to change the air filter regularly. More so in dusty conditions.
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Riding Mower Won't Start Without Spraying Gas into Carburetor