Troubleshooting a gas lawnmower engine isn't difficult. The first thing to do is to remove the spark plug and make sure you have spark. If you have a spark and still have trouble getting the lawn mower to start, make sure you have gas. If you have spark and gas, the problem could be a blocked fuel line or fuel system. If the fuel system is clear, you may need to take your carburetor in for service. Checking and clearing your fuel line is not hard.
Put on eye protection. This is an essential step. When working with a lawn mower's gas lines, gas may sometimes spray out and get in your eyes.
Disconnect your spark plug before performing any service on your lawnmower.
Unscrew the gas cap. Make sure nothing is blocking the cap. Lawn mower gas caps are designed to allow air into the tank to prevent vapor lock. Vapor lock will prevent gasoline from flowing from the tank into the fuel line. Check to see if someone has added plastic or another sealant to the bottom of the cap. If it has, remove the plastic or sealant and try to start the engine. If the engine starts, you have cleared the block.
Drain your gas tank by disconnecting the gas line from the carburetor. Drain the gas into a pan or gas can.
Look for a fuel filter on your lawn mower's fuel line. The fuel line is a clear, plastic hose that runs from your gas tank to your lawn mower's carburetor. If you have a fuel filter, replace it. It is very difficult to visually check for a filter blockage, so replacing the filter is the best way to ensure that it is clear.
Replace the fuel line when replacing the filter. Fuel lines are not expensive and you must remove the lines to remove the fuel filter. Replacing both the lines and filter will ensure that there is no blockage in this part of your fuel system.
Look inside your gas tank. Many tanks have a screen over the fuel outlet to the gas line. Ensure that this screen isn't blocked. If it is, remove the screen and replace it with one from the lawn mower manufacturer.
Once you have replaced the lines, filter, and ensured that there is no blockage in the tank itself, connect the gas lines and filters to the tank, but not to the carburetor. Place your hand over the tank opening creating a seal. Blow through your hand to ensure that air can pass unobstructed through your fuel lines.
Connect the gas line to your carburetor and fill the tank. Unless you have a problem with your carburetor, your lawn mower should start.