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How to check a lawnmower sparkplug

One fine day, you get up and realize the lawn has not been done lately. You have your morning coffee, take your breakfast, and get ready to trim the exceedingly grown grass. You grab your equipment and give the mower a pull. At this moment, you realize that the mower is not picking up the ignition. It gets you very upset. Your high aims train has now been derailed. The very obvious blame lies on the faulty sparkplug.

But behold, before putting all the fault on the shoulders of the sparkplug. It might be that any other component in the ignition system might be faulty. It is good to check all parts before changing or repairing anything. Here we will give the old sparkplug a test. Or maybe there is a sparkplug that lies in your backyard, and you want to check whether it is any good.

How to check a lawnmower sparkplug

  • Step 1: Remove the sparkplug wire
  • Step 2: Remove the sparkplug
  • Step 3: Give the sparkplug a visual check
  • Step 4: Clean the electrodes if they are carburized
  • Step 5: Use a Digital multimeter to check continuity
  • Step 6: Check the leaks of current using DMM
  • Step 7: Check the resistance range if the sparkplug has a resistor
  • Step 8: Reinstall the sparkplug
  • Step 9: Use sparkplug tester to check the spark production

Checking the sparkplug is just one of the three significant steps to repair the lawnmower when it fails to start.

What is a sparkplug?

Defining the construction and working of sparkplug is necessary to give the repairer better insight. When you know the physics and mechanism behind sparkplug behavior, you will be more interested in learning its condition. Checking the sparkplug is not a difficult task but a critical procedure to know for a lawnmower owner

– Detailed construction

Here all the different components of sparkplug will be discussed.

  • Central electrode: The central electrode conducting current from the ignition coil and is present throughout the center of the sparkplug
  • Ground electrode: The side-mounted electrode that conducts the spark back to the ground
  • Electrode gap: Present between both electrodes through which high voltage current jumps and generates the spark
  • Copper core: Deeply inserted in the center electrode for better heat conductivity providing a complete range plug that works well at both high and low speeds
  • Resistor: Present in models with an” R” in their label and provides resistance to the high voltage current to save the electrode from blowing up
  • Gasket : Ensure the pressure is sealed inside the engine cylinder
  • Metal shell: Zinc-plated and chromated to guard against corrosion
  • Insulator: Made of high purity alumina, providing high electrical insulation, better heat dissipation, and stronger thermal shock resistance
  • Special packing: Excellent air tightness and robust construction
  • Part number: Imprinted on the ceramic portion of all sparkplug might vary from manufacturer to manufacturer for the same type of plug
  • Corrugations: To prevent the flashover. Flashover is the conductance of high voltage current directly to the ground instead of producing a spark through the gap
  • Terminal: The point where the sparkplug wire is connected that allows the current from the ignition coil to the sparkplug

– Working of sparkplug and ignition system

The ignition system is the electro-mechanical mechanism that provides initial power to the machine to start.

  • Battery: It provides a source of electricity. Keeps the charge stored and gets charged through the generator that runs from a belt transferring power from the crankshaft
  • Starter motor/ rope: The initial push that allows the engine to perform the first intake and compression
  • Flywheel: The transmission from the flywheel that is turned to move the piston in the cylinder as required
  • Ignition coil: Acts like a transformer and steps up the 12 V from the battery to high voltage for spark production
  • Relay: Allows the current to floe with specific timing when the spark is required inside the cylinder during the power stroke
  • Sparkplug: The component that is directly in contact with the combustion chamber and generates a spark to burn a fuel-air mixture and produce power

All these components work together and make up the ignition system. After starting, the starter rope and flywheel do not have to do any work; however, the rest of the cycle continues and keeps the engine running.

Step by Step guide to check the sparkplug

● Step 1: Remove the sparkplug wire

After locating the plug, removal of the plug wire is performed. The wire can be pulled off using hands. It is preferred to use a sparkplug wire puller, ensuring the insulation does not get damaged.

● Step 2: Remove the sparkplug

If you want to remove the sparkplug, either a socket wrench or a sparkplug wrench tool can be used. The specialized tool has an extended neck of the socket and makes the job much more comfortable. Follow these stepwise instructions

  • Fit the wrench in place: Connect the wrench to the base of the plug
  • Loosen and unscrew the plug: Apply firm force to loosen up the sparkplug and then unscrew with ease with normal hand force
  • Removing old sparkplug: When unthreaded completely move the old sparkplug out

● Step 3: Give the sparkplug a visual check

Three things can be checked visually in a sparkplug

  • Ceramic insulation: check if there are any cracks in the white ceramic part of the sparkplug. If it is damaged, then most probably the plug is useless as it will be flashing over
  • Electrodes: check whether both of the electrodes are still present because sometimes they might blow up. In this case, it is a wise decision to throw it away.
  • Terminal: Check for any carbon deposits on the terminal side. It is sporadic to occur but easily observable.

● Step 4: Clean the electrodes if they are carburized

If there are carbon deposits on the electrode surface, it is better to clean them first to observe the best checking readings.

  • Use a soft wire brush
  • Brush through the neck thread and sides first
  • At last, give a gentle brush to the electrode
  • It is advisable not to use any abrasive methods like sanding or grinding as it will cause permanent damage to the plugs

● Step 5: Use a Digital multimeter to check continuity

  • Set the DMM in the current mode
  • Touch the probes together to check for proper functioning
  • Put the sparkplug on an insulator surface (wood or cloth) and not on a metal vise or floor
  • Touch one probe to the tip of the central electrode and the other to the terminal
  • The DMM should give a beep and offer a 0 reading
  • It shows that the central electrode is not broken 

● Step 6: Check the leaks of current using Digital Multi Meter

  • Step 1: Now instead of touching the other probe to the central electrode, connect it to the ground electrode
  • Step 2: The open-loop error should be visible
  • Step 3: Touch the probe on the gasket, insulation, and hex nut
  • Step 4: All should have an open-loop
  • Step 5: If the current conducts, the sparkplug is bad

● Step 7: Check the resistance range if the sparkplug has a resistor

If the spark has a resistor, It will have a letter R in the imprinted code. Instead of zero reading, a value of resistance will be observed. If this resistance is from 4-7 ohm, the sparkplug is okay. If higher or lower reading is followed, the sparkplug is terrible. Some plugs have a regular reading of 9 ohms, but in that case, it is generally mentioned as the last digit in code, i.e., 9.

● Step 8: Reinstall the sparkplug

  • Mind the electrode gap: Before reinstalling the sparkplug, it is imperative to gap it. Using the gap gauge or feeler gauge, choose the specific gap dimension a push between the electrodes.
  • Place the sparkplug in place: After gaping is completed, gently screw the new plug into the initial thread. Be as gentle as possible so that threads are not damaged.
  • Tighten the sparkplug: Screw with hand force and tighten using a wrench tool. Make sure not to overtighten because it might damage the plug, its seal, and threads

● Step 9: Use sparkplug tester to check the spark production

  • Installing the tester: Install one end to the terminal of the sparkplug, and the other end is connected to the connector wire
  • The check step: Pull the starter rope. A spark will be produced inside the transparent chamber. It proves the sparkplug is good

If the sparkplug passes all these tests, it is good enough to work. If the engine does not start, you still need to move one and check other ignition and air intake parts, i.e., carburetor.

Related Question

1. What to do to make sure the spark plug does not stick when changing next time?

Using an anti-seize compound around the thread of the sparkplug before threading is essential.

2. Is there any more straightforward method?

Yes, using a new sparkplug instead of the old one is a method to check. If the engine starts by doing this, this is a test that the older sparkplug was terrible. 

Final Remarks

Sparkplug is an essential component for the machine to start. Knowing how it works and what parameters are to test it for its performance are discussed here. Every lawnmower owner or any garden equipment should have detailed knowledge of sparking and sparkplug condition testing.