Spark Plug for Generators

Do you own a generator?

A spark plug is an important part of your generator, and it’s crucial to keep it in good condition. If your generator isn’t working properly, the spark plug might be the issue.

Follow these simple steps to service your spark plug and keep your generator running smoothly. It’s easy – even if you don’t have any prior experience – and it only takes a few minutes.

Read through the full article below to learn more about how to service your spark plug!

How Does A Spark Plug Work?

Spark plugs are critical components of the ignition system of any portable generator. The ignition system provides the spark inside the combustion chamber to start your small engine, whether you tug the rewind rope or use an electric start.

The ignition system is made up of

  • an ignition armature with copper wire windings mounted adjacent to the flywheel,
  • magnets placed on the surface of the flywheel,
  • a spark plug and a spark plug lead connected to the armature.

When you rotate the flywheel by pulling the rewind rope, the magnets pass the ignition armature winding inducing an EMF with each rotation. This EMF results in an electric current that produces a high-voltage spark at the spark plug’s tip. The system is coordinated such that the spark is only produced when the piston is at the point of maximum compression of the air-fuel mixture for optimum performance of the engine.

Modern solid-state ignition systems require about 10,000 to 20,000 V to generate the spark across the spark plug gap. A tiny transistor closes the electrical circuit as the magnets approach to convert 2 – 3 A of generated current into these high-voltages that are fed to the spark plug through the spark plug leads.

Servicing Spark Plugs (SP)

If the spark plug electrodes are not clean or sharp, you will not get the powerful spark needed for ignition. Your generator will show signs that the spark plug needs inspection, cleaning, adjustment, or replacement.

In general, the dirtier or worn out a spark plug is, it will require a higher voltage or a tug with more force to produce an adequate spark. If you have pulled the rewind rope multiple times, this may indicate a damaged spark plug.

In some cases, you may experience sluggish engine operation or poor acceleration. This is caused by “spark miss,” which is a term used for inconsistent firing by the SP. The consequences of these problems are higher fuel consumption, oil dilution, and deposits on the cylinder head.

Cleaning and Inspecting a Spark Plug

Spark plugs are one of the easiest parts of the engine to inspect and repair. All you require is the following set of tools:

  • spark plug socket, depending on the size of your spark plug,
  • socket wrench
  • spark plug gauge
  • spark tester
  • plug point cleaner, and
  • wire brush

The following steps will guide you to inspect and clean the spark plug of your generator.

  1. To begin with, disconnect the spark plug lead and clean the area around SP. This ensures that no debris will enter the combustion chamber after removing the plug.
  2. Use a spark plug socket to remove the spark plug.
  3. First, clean the lighter deposits with a wire brush. Use a spray-on plug cleaner if required. If tough deposits are observed, use a sturdy knife to scrape them off. Do not use any form of an abrasive on the plug.
  4. If you find burnt or worn electrodes, cracked porcelain, or very stubborn deposits, you may have to go for the replacement of the spark plug.
  5. Use your spark plug gauge to measure the gap between your spark plug’s straight and curved electrodes.
  6. You may adjust the gap slightly by gently bending the curved electrode.
  7. Check the gap again with the gauge, and if correct, reinstall the plug and attach back the spark plug lead. Take care not to over-tighten the plug.

Checking Ignition with a Spark Tester

A spark plug tester allows you to easily locate problems with the ignition system. The testing with the spark plug tester involves the following steps:

  1. Connect the spark plug lead to the tester (long terminal). Connect the tester to the ground at the engine body using the alligator clip that comes with the tester.
  2. Crank the engine using the electric start system or the rewind rope. Look for a spark in the tester’s window.
  3. If you can notice the spark jump the gap in the tester, your system is okay. If the test results are not satisfactory, you may have to remove and inspect the plug as in the previous section.

Checking for a Spark Miss

Testing for a spark miss by the following method will tell you if the spark plug is firing erratically.

  1. Connect the long terminal of the spark tester to the spark plug lead. Connect the alligator clip of the tester to the spark plug screwed into the cylinder head.
  2. Start the engine and observe the gap in the tester for some time. Look for an uneven timing of sparks in the tester which will indicate spark misses.

In addition, your engine may show an appreciable reduction in sound, black smoke or a popping sound. This is due to unburnt fuel getting exhausted that may ignite while passing through the muffler.

Replacing the Spark Plug

If after thorough inspection of your spark plug you have decided to replace it with a new one. The following tips will help you in selecting the best spark plug for your power generator.

Look Into The Owner’s Manual

Start by looking for the required information in the owner’s manual that came along with your machine. This information is usually available in the maintenance section with some manufacturers even recommending a brand and the model number for the spark plug. Others restrict themselves to the details of the spark gap, materials and the socket size.

If no specific make is recommended by the manufacturer, it is always better to purchase a high quality product from a reputed manufacturer as this will have a huge impact on the performance of your device. Some of the reliable brands are NGK, Denso, Champion, Autolite, Bosch, ACDelco, Troch, etc.

Mind The Required Spark Plug Gap

You must maintain the equivalent gap between the two electrodes – one curved and the other straight located at the tip of the plug. Most of the portable generator machines have spark plugs with the

  • gap size: 0.028″ – 0.031″ (0.7 mm – 0.8 mm), and
  • socket size: 5/8″ (16mm) or 13/16″ (21mm).

You will require a socket and ratchet of the above sizes to replace the spark plug by yourself. As stated earlier, you should check the above with your user manual or the existing spark plug.


That’s all there is to it! Now that you know how a spark plug works, what the recommended gap distance is, and how to service your spark plugs, go ahead and get out there and change them yourself. It’s an easy process that will save you money in the long run – not to mention, it gives you a feeling of accomplishment when you do it yourself. Let me know in the comments section if you have any questions or need any help getting started.

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