How To Check A Lawn Mower Coil?

How To Check A Lawn Mower Coil

In this post, I will be answering your question of how to check a lawn mower coil!

The use of lawnmower engines is to power lawnmowers, tillers, blowers, go-carts, and many other common items. The engines can last a long time if they are taken care of. Otherwise, engines slowly lose their power and ability to run smoothly. If the engine stops working accurately, it is necessary to take the engine apart and replace worn parts. This project is very time-consuming and requires patience and a few important tools. 

The electrodes on a spark plug should be clean and sharp to produce the powerful spark required for ignition. The more worn spark plugs are, the greater the tug on the rewind. That is why more effort is required to produce an adequate spark. If you haven’t turned on your engine recently, your mower won’t get started.  

You have to tug repeatedly on the rewind to start the engine; a damaged spark plug may be the culprit. These problems could be excessive fuel consumption, deposits on the cylinder head and oil dilution. Among all the engine components to replace, spark plugs are one of the easiest and most inexpensive. 

Reading the engine and equipment manual(s) is a must before starting, operating, or servicing your engine or equipment to avoid personal injury or property damage. Find all engine safety warnings. If you have more questions or you are unsure of the procedure, then contact the authorized technician of the company:

How To Check A Lawn Mower Coil

How Do Ignition Systems Work in Lawn Mowers?

It won’t be wrong to call your ignition system the starting system. To produce a spark inside the combustion chamber, you have to rely on an ignition system, whether you start the engine with a pull rope or the turn of a key on an electric start motor. 

Parts of an ignition system of a small engine

  • Flywheel with magnets
  • Coil or Armature
  • Push Button starting or Pull Cords (depending on your engine type)
  • Spark Plug Lead
  • Spark Plug

When you start your lawnmower or small engine, you switch the flywheel, and its magnets pass the coil (or armature). That leads to a spark. The ignition system coordinates the timing so that the spark will ignite the air-fuel mixture in the combustion chamber. It reaches maximum compression in each engine cycle and, hence, maximises the engine’s power. The flywheel keeps rotating, the magnets keep passing the coil, and the spark plug keep firing based on a specific timing as soon as the engine starts running. 

Types of Ignition Systems

  • Solid-state systems.
  • Breaker point systems.

Common Flywheel Problems

If you are facing ignition timing issues, this is most often because of a sheared flywheel key. Flywheel magnets can also be tested for any potential issues.

Common Spark Plug Problems

  • Depending on the temperature, altitude and engine settings, the required spark plug gap and voltage can be changed. 
  • Old, damaged or fouled spark plugs can also demand replacement service.

When To Check & Replace Spark Plugs

For standard lawn mower and small engine maintenance, you should replace the spark plugs:

  • Every year and in accordance with the manual.
  • If your lawn mower or outdoor power equipment is not going to start, replace the spark plugs.

How to Test the Ignition Coil in Your Small Engine

Safety Warnings

Whenever you attempt to start the engine or equipment, stay clear of any rotating, moving parts or other hazardous areas.

The coil is probably the easiest and the foremost thing to check.

Testing the Coil or Armature

  • Clip one end of the spark tester to the ignition cable and the other grounded to the cylinder head. 
  • Spin the flywheel rapidly (at least 350 RPM). Watch for the spark in the tester window. If the spark jumps the tester gap, your ignition coil is working well. If not, it is to be replaced. 
  • Does your engine quit while running? Hook the tester up between the ignition cable and the spark. Start the engine. When it stops, you better monitor the window.

Common Mistakes When Testing Coils

  • Be sure to unhook the coil from the equipment wiring harness and the engine’s wiring harness. Use the spark tester. Often, a technician is fooled into replacing a good coil because the coil grounding lead was shorting out against a piece of sheet metal. 
  • Don’t attach the tester to the spark plug for this test at any cost. The engine could start. Without the grounding lead installed, you won’t be able to switch it off.
How To Check A Lawn Mower Coil

Replacing Ignition Coils

An ignition armature must be set at a fine distance from the flywheel. Your engine repair manual will provide you with the proper gap for the engine. Common armature gap ranges are .006 – .010″ and .010 – .014″. Armatures are often packaged with a shim to help in setting the gap. Index cards of the proper thickness also work nice. 

  • Unhook the spark plug wire and secure it. Removing any batteries if equipped.
  • After that, remove the old ignition coil (armature) mounting screws.
  • Afterward, disconnect the stop switch wire from the flywheel brake. Remove the coil.
  • Attach a replacement coil from the original engine manufacturer with the help of mounting screws.
  • Push the coil (armature) away from the flywheel. Tighten one screw.
  • Turn the flywheel so that the magnets are on the opposite side of the ignition coil.
  • Place the apt shim between the rim of the flywheel and the ignition armature. While you are holding the shim, turn the flywheel as long as the magnets are directly adjacent to the armature. 
  • To make the magnets pull the ignition armature against the flywheel and shim, loosen the screws. Then, tighten both mounting screws and rotate the flywheel. The shim will slip free. 
  • Reconnect the stop switch.


How do I know if my lawn mower spark plug is bad?

There are a few clues that it’s time to change a spark plug. The mower is more complicated than normal to start. Poor Engine Performance is another clue. Excessive Fuel Consumption is the symptom of the poor plug. Physical Appearance is significantly changed.

What happens if you use the wrong spark plug in a lawn mower?

The engine will not start possibly. If it does, using too cool of a plug may cause it to foul, either causing the engine to stall or make future starts after the initial, difficult.

Will any spark plug work in a lawn mower?

There are different plugs for different engines. Usually, it would be best if you had the engine model on your mower. Take in the old spark plug to the hardware store. There is a booklet at most of the stores showing your old model of spark plug and what model the brand in front of you is compatible with.

Can spark plugs cause a lawnmower not to start?

A lawnmower may not start if the spark plug is dirty or fouled.  It can also work itself loose, and that may cause issues later. If the spark plug appears to be seated correctly, but the engine doesn’t start, a new one can be in order. This easy fix can get your small engine working again just for a few dollars. 


An ignition coil is an integral part of the ignition system. If it is damaged, your engine will not start properly. We have discussed some valuable steps that you can follow to check your coil. Make sure you follow all safety instructions to keep your lawnmower and yourself safe. 

Noah Burns

I am Noah Burns- The Guy behind I will Make You Smart. I love to experiment and test new products with an aim to create informative contents for readers like you. It is my aim to make this site a leading source of information and reviews to help consumers make more informed buying decisions.

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