The carburetor on virtually every lawn mower infuses gasoline into the engine, whether it is a 2-cycle mix with oil or a 4-cycle mix of straight gasoline. This gasoline, plus any dirt or debris that makes its way past the air filter, can cause a gummy, sticky substance much like varnish that will affect the proper function of a carburetor. At first this buildup may just cause the engine to run poorly, but eventually it could cause major carburetor problems that, if not taken care of before it's too late, might cause an expensive carburetor rebuild.
Remove the air cleaner top by unscrewing the screw that holds it in. When you pull the screw out, the cover will come with it.
Pull out the air filter element from inside. It will be a spongy type of material, and it will just lift up and out.
Start your mower as you normally would, and let it run for a few minutes until it is warmed up.
Push the throttle lever to about two-thirds power.
Hold the stop lever handle securely in one hand so the lawn mower doesn't die; it needs to keep running. Shoot a one-second spray of carburetor cleaner into the carburetor where the air filter was. The cleaner will get sucked through all of the jets and past the needle valves inside of the carburetor, dissolving all the gunk it touches.
Shoot another one-second spray into the carburetor for good measure. Allow the motor to run for a bit to burn up any cleaning spray.
Shut the motor down, and reassemble.