When you’re staring at a yard full of grass that desperately needed cutting two weeks ago, the last thing you want is a lawn mower engine that won’t fire up.
Before you have a heart attack pulling on the rip cord of your lawn mower (or snow blower), check the fuel and carburetor. They’re the root causes of more than 80 percent of all no-starts. I’ll walk you through the steps.
You’ll need hand tools and a socket set, a can of carburetor cleaner and your air compressor. And you’ll probably have to make a trip to the small-engine parts store. But after an hour of effort, you just might have an operational engine, and you’ll save a bundle by fixing it yourself. Let’s dig in.
I’ll assume you’ve cleaned the air filter, so the next step is to remove the spark plug to see if it’s wet. If it is, there’s no way the engine will start. So clean the plug with carburetor cleaner and let it dry. Cleaning it with compressed air isn’t enough; you need a solvent to remove oil residue. If the plug was wet, move on to Step 3. If it was dry, skip to Step 4. If the fuel is more than a month old, dispose of it properly and refill the tank with fresh gas. Then reinstall the spark plug and try starting. It may take quite a few pulls to suck the new gas into the carburetor, so be prepared to clean and dry the plug a few more times.
An anonymous reader from Oakland, ME took issue with my statement that finding corrosion in a small engine carburetor means "game over." Instead he advises boiling the disassembled carburetor in vinegar for 30 minutes. He says the pitting will remain, but the vinegar will remove the rest of the corrosion. Since vinegar is cheap, you've got nothing to lose but time.
The engine can’t get gas if the fuel filter is plugged or the carburetor inlet needle is stuck. Check the fuel filter (if equipped) by removing the fuel line at the carburetor. Gas should run out. If it doesn’t, remove the fuel line ahead of the fuel filter inlet. If gas flows, the filter is clogged. Replace it. If you still don’t get any gas, the fuel line is kinked or plugged. And check inside the tank for any debris that might clog the outlet.
If you’re getting gas to the carburetor, check to see if there’s any fuel in the bowl. Clamp off the fuel line with a C-clamp. Then remove the bowl (Photo 1). If the bowl is empty, the problem is a stuck inlet needle and seat. They’re easy and cheap to replace (Photo 2). But before you buy the parts, check the condition of the rest of the carburetor’s interior (Photo 3). If you see any corrosion, it’s “game over.” A corroded carburetor is a dead carburetor. Replace it.
A clogged main jet is a pretty common problem. You can try cleaning it with spray carburetor cleaner (Photo 4). Then try starting. If the engine still isn't getting gas, replace the carburetor.
If you got the engine to start by cleaning the main jet, but it runs rough or the idle speed surges, you have two choices—rebuild or replace the carburetor. For rebuilding instructions, see “How to repair small engines.” Otherwise, disconnect the old carburetor from the linkage, remove the two retaining bolts and slap on a new carburetor.