Perhaps your gas powered lawnmower has sat for several years, and has since been contaminated with water condensation, or perhaps you put in the wrong type of fuel. Either way you are in need of getting gas out of the engine. This is a quick and easy fix: Take a quick trip to your local hardware store and buy a simple hand pump siphon—essentially two pieces of tubing attached to a hand held pump which is open on both ends, allowing liquids to flow through it.
Make sure you have a receptacle for the gas to be siphoned into—an old gas can will work perfectly—if not available make sure your receptacle can hold the amount of liquid needed to be removed, is without any leaks, and has a secure top. Place the lawnmower on a flat surface that has it sitting above the height of the gas can—this will aid in the siphon’s force and flow. Submerge one end of the siphon tubing into the lawnmower, below the height of the gas, and the other into the gas can. Begin hand priming the pump to start the flow of gas from the lawnmower into the tank—if enough suction can be generated, a steady flow of fluids will be achieved as long as the tube in the gas tank remains submerged. If not enough fluid is present and a stream cannot be achieved, albeit laborious, you can hand pump the entirety of the contents from the mower to the can.
It is not recommended to start a siphon by sucking on the tube, as the accidental ingestion of gasoline can have very serious repercussions. However, get a long piece of clear tubing. Put the mower on the table, a gas can on the floor, and place one end of the tube into the gas tank on the mower. Allow the tube to droop down below the tank, and back up above the tank, creating a U—at the top end of the U, begin sucking the gas out, when you see it extend to the bottom of the you, quickly take the free end and place it into the gas tank, until the contents are emptied from the tank.