You keep pulling the starter rope on your gas-powered string trimmer until it feels like you’ve dislocated your shoulder, but the engine won’t fire up. Or, perhaps, you’re turning the ignition key on your lawn mower only to hear a brief straining whine before… silence. The cause could be a defective spark plug, a malfunctioning ignition coil, a restricted carburetor, a clogged fuel filter, or something else entirely.
To get your outdoor power equipment up and running as quickly as possible, you’re going to need to identify the source of the problem. One way to do this is to test the engine’s ignition system. This can be done by using an ignition tester, a relatively inexpensive accessory that should be part of every tool box, whether you’re a professional landscaper or a do-it-yourself backyard enthusiast. An ignition tester will speed up the troubleshooting process by allowing you to quickly determine if the spark plug or ignition coil is faulty on most two-cycle and four-cycle engines.
Remind me about the difference between a two-cycle and a four-cycle engine?
Larger outdoor power equipment, such as lawn mowers, snowblowers, pressure washers, wood chippers, and more heavy-duty tillers will depend on four-cycle engines to provide the necessary torque, whereas smaller hand-held equipment, such as string trimmers, chainsaws, pole pruners, and leaf blowers, can get by with the lesser amount of torque provided by a two-cycle engine. The main difference between the two types of engines is that the four-cycle engine requires two revolutions of the crankshaft and piston to draw the fuel and air into the cylinder, ignite it, then exhaust the combustion gases; the two-cycle engine requires just one revolution to do the same thing.
Additionally, a larger four-cycle engine may utilize an electric start feature that allows the engine to be turned over by using an ignition key or a start button. Smaller four-cycle engines, and all two-cycle engines, rely on the operator to pull a starter rope to fire the engine up.
How a small engine ignition system works
When that start button is pressed, the ignition key turned, or the rope is pulled, the starter turns the flywheel which, in turn, rotates the crankshaft. The rotating crankshaft connects to the piston which moves up-and-down within the cylinder and ignition process begins. The flywheel has permanent magnets built into it, and as it rotates past the ignition coil, a magnetic field is created. The magnetic field induces electricity, allowing the ignition coil to send voltage to the spark plug.
On both four-cycle and two-cycle engines, fuel and air will enter the cylinder where it is compressed by the piston as it travels to the top of the cylinder. The spark plug fires and ignites the fuel and air mixture. This drives the piston back down through the cylinder to rotate the crankshaft. The momentum of the spinning flywheel provides enough force to push the piston back up where the spark plug will again ignite newly drawn in fuel and air. This process continues with every revolution of the crankshaft until the engine is shut off.
How to test the ignition on a small engine
The number one cause of ignition failure is a spark plug that has degraded due to carbon build-up and/or a weakened electrode. Because of this, it is recommended you replace the old spark plug with a new one at least once a year as part of your annual engine maintenance. The ignition coil could also be malfunctioning. Both of these components can be tested with an ignition tester to determine if they’re functioning adequately or not.
To test the engine’s spark plug and ignition coil:
- Remove the spark plug wire boot from the spark plug terminal.
- Attach the ignition tester clip to the spark plug terminal and insert the other end of the tester into the wire boot.
- Now attempt to start the engine (when doing this, make sure you stay clear of the tester and any electrical components to avoid a shock).
- If a spark jumps across the terminals in the tester window, or the engine starts, the spark plug and ignition coil should be working properly.
- If you do not observe a spark, unclip the tester from the plug and attach it to a good grounding location such as a bolt or the valve cover.
- Attempt to start the engine again.
- If you now observe a spark in the tester window, the spark plug is probably worn out and will need to be replaced.
- If there is still no spark in the tester window, it’s likely the ignition coil is defective or there may be another problem with the equipment’s electrical system.
Using the tester to determine if the ignition coil is faulty or another electrical component is:
- Disconnect the grounding wire from the ignition coil.
- Attempt to start the engine again, but make sure the tester remains clipped to a grounding location (valve cover or bolt) and not the spark plug.
- If you observe a spark in the tester window, the ignition coil should be functioning properly, so there may be another failure in the equipment’s electrical system.
- If no spark is visible in the tester window, it’s likely the ignition coil has failed and will need to be replaced.
If your troubleshooting has revealed you need a new spark plug, ignition coil, or another component, your next step is to find the right part that matches your small engine. As your repair partner, Repair Clinic.com takes the guesswork out of your search. Just type the full model number of the engine in the Repair Clinic website search bar to see a complete list of compatible parts. You can then use the “Part Category” navigation filter (“Ignition Controls”; “Spark Plug”) to identify the specific part that fits your engine, whether it’s a spark plug, ignition coil, primer bulb, air filter, fuel filter, or carburetor. Repair Clinic stocks genuine manufacturer parts for all the big names in small engines, such as Briggs & Stratton, Craftsman, Honda, Husqvarna, Kawasaki, Kohler, MTD, Snapper, Tecumseh, Toro, Walbro, and Zama, and can ship these parts to you quickly.
But wait, there’s more! Repair Clinic is home to the internet’s largest library of free repair help: thousands of step-by-step guides, schematics, and videos showing you how to troubleshoot your outdoor power equipment, home appliance, or heating and cooling system problems, replace that defective part, and maintain your products cost-effectively. Join the long list of satisfied customers who are doing-it-for-themselves with help from Repair Clinic.