The function of the ignition system is to create a spark on the plug and Ignite the fuel/air mixture in the cylinder. A device called Magneto generates high voltage, and any of the mechanically operated points or an electronic switch will trigger the spark at the appropriate time in the four-stroke cycle.
More than 90% of the time, problems with gas engines are due to a carburetor problem. However, the spark plug is easy to check first.
See the ignition system diagram below.
The Magneto
On a push mower and other battery-free motors (such as chainsaws, trimmers, and motorcycles without batteries), a device called Magneto is used to generate the high-voltage spark. Magnets are even used on aircraft with a piston motor for safety reasons, so the failure of the battery or alternator does not cause the engine to cut. A magneto is a sort of crossover between a generator and a transformer and has a primary and secondary coil.
The magnet is mounted by the steering wheel (the thing with fins on top of the mower, which rotates). A magnet embedded in the edge of the flywheel induces a current pulse on the primary coil as it moves rapidly beyond the magneto poles. When the current reaches a peak, a set of switch contacts called points is opened, interrupting the current. This causes the magnetic field in the primary coil to collapse rapidly, causing a voltage spike in the secondary coil. This coil, which has many cable turns, increases the voltage to approximately 10,000 volts (or more). A capacitor, also known as condenser, absorbs the current produced by the primary coil allowing the magnetic field to collapse rapidly. The modern engines have electronic ignition and the points are replaced by an electronic switch called thyristor or SCR (silicon-controlled rectifier).
- Some engines (such as those used in the ride mowers) do not have magnets, and instead will use an on-board battery and a ignition coil to produce a spark, similar to a car's system.
- Schematic of lawnmower ignition circuit with mechanical points. The points are connected to the primary of the magneto, the spark plug to the secondary.
- Diagram of the lawnmower's ignition circuit with mechanical points. The dots are connected to the magnet's primary, the spark plug to the secondary.
- Modern engines have electronic ignition and there are no points to clean. If the ignition module fails, the complete unit needs to be replaced.
- Modern engines have electronic ignition and there are no points to clean. If the ignition module fails, the entire unit must be replaced.
- Engine flywheel, visible when the cowling or cover over the mower is removed.
- Motor flywheel, visible when the fairing or cover is removed on the mower.
- Flywheel, magneto, and governor vane.
- Steering wheel, Magneto and governor's paddle.
- Spark Test on Plug
- First, verify that the wire that feeds high voltage to the spark plug should not be loose and must be snug. Sometimes the cable may come out of the rubber sheath and the terminal that pushes toward the spark plug. Then, using a spark plug wrench, remove the plug, replace the plug cable into the plug, and place the threaded part of the plug in contact with the motor block to ground it.
- Now pull the starter cable and look for a spark. You must keep the plug tight against the motor in some way, as it may bounce when you pull the starter cord. This is where an assistant can be useful as it can be difficult for you to see the plug from where you are pulling the starter cable. Try pressing the plug against the lawn mower with a plastic piece or similar, such as a plastic garment peg. The cable could hold the plug, but if it is wet or if the insulation is bad, you may end up getting shocked!
- If you do not see a spark at this point, there may be a crack in the stopper insulation or it may be dirty and should be cleaned with an old toothbrush and a little gasoline. A wire brush the size of a toothbrush is even better. Let it dry and try to light a spark again. If you are not successful, try another plug and check again. It does not hurt to have a replacement plug for this purpose or to use a car plug, but do not use this plug on the motor unless it is the correct type, as it could hit the piston and cause damage if it is too long.
- The external electrode of the plug must be filled with a small file if it has been rounded. The spark plug space should usually be adjusted to 0.030 "(30 thousandths of an inch)/0.75 mm, but check your engine manufacturer's recommendations. The separation is measured by a thickness gauge (see photo below). These are generally in the form of a set of steel strips of varying thickness of approximately 0, 002 inches to 0, 030 inches.
Caution: The threads on a plug are steel, but the engine block and cylinder head are usually aluminum alloys, which is smoother than steel. So that the threads can be damaged if you are not careful. Check for dust or other particles on the plug or motor threads before screwing. Make sure the plug does not kink and do not overtighten. Ideally, you must use a torque wrench to tighten the cap or when you replace the blade on a lawnmower.
If you damage the threads and the plug does not tighten, you can obtain a Helicoil installed in a repair shop. This involves hitting the stock and screwing an insert into place that has internal threads to fit the plug.