Warm weather is here, and you decide to use your gas lawn mower to trim your lawn. So you change the oil (if it’s a four-stroke engine), fill up the tank with gas, set the throttle, and give the starter a pull. But, your gas lawn mower only responds with a sputter. As long as the spark plug is delivering power to the engine, it’s time to troubleshoot a gas lawn mower carburetor problem.
The first step, before you even look at the carburetor, is to check the gas in the tank. If it’s fresh, then that’s probably not the problem. But, if there is any old gas in the tank leftover from last season that could be the reason your lawn mower won’t start. Gasoline turns to a varnish-like substance when it’s stored too long. If this is the case, you’ll need to empty out the gas tank of your lawn mower.
If the gas tank comes off the mower easily, you can simply remove it and empty the old gasoline out of it. Otherwise, you’ll need to siphon it out. You can buy a siphoning device at your local auto parts store.
Then, the next step is to refill the tank on your lawn mower with fresh gasoline. If there’s still gunk in the gas tank that you couldn’t get out, you’ll also need to buy a fuel system cleaner. This is an additive you can also buy at your local auto parts store.
(To avoid having problems that old gas can cause with starting your lawn mower engine, before you store it away for the winter, drain the gasoline out of the tank. Or, add a fuel stabilizer to the gasoline.)
Try to start your gas lawn mower again. If the engine still won’t run, or if it starts, but it sputters and stalls, you’ll need to continue to troubleshoot the problem. A small engine, like the one that operates your gas lawn mower, needs to have the right mixture of fuel and air. So, check the air filter. Make sure it’s clean and installed properly.
A paper air filter needs to be replaced once it becomes dirty and clogged. A foam air filter, on the other hand, can be cleaned out with warm, soapy water. Rinse it clean, then give it a gentle squeeze to remove the excess water. Squeeze the air filter dry with an old rag, or allow it to air dry before you put it back in.
Now, try to pull start your lawn mower again. If the engine still won’t start, the problem probably lies with the carburetor, so you’ll need to check that.
The next step is to locate the air adjustment valve on the carburetor. Carefully turn the valve to the right as far as it will go. Now, turn the valve back to the left about two turns. Give the pull start another try. if the engine finally starts, don’t get excited, because you’re still not quite done troubleshooting the gas lawn mower carburetor problem. You’ll still need to adjust the amount of air that’s entering into the carburetor.
If the engine sputters and idles roughly, turn the air adjustment valve a bit farther to the left. Or, if that doesn’t remedy the problem, try turning it to the right instead. Once the gas lawn mower engine is running smoothly, you’ll know you have adjusted it correctly.
If the air adjustment valve on the carburetor wasn’t the problem afterall, you’ll need to continue to troubleshoot the problem. It may be that the line that delivers gasoline from the tank to the carburetor is plugged up with gunk. If this is the case, then the engine isn’t getting any fuel to run on.
The next step, then, is to locate the air breather. Remove the air breather and watch inside the carburetor while you give the pull start another yank. If you can see gasoline, then the fuel line isn’t plugged up.
If your gas lawn mower still won’t start, you’ll need to check your owner’s manual or take it to a repair shop.