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How I Read My Motorcycle's Spark Plugs

By Jim Noss

This past weekend, I installed new spark plugs in my Toyota truck. So while it is fresh in my mind I wanted to cover the basics of how to read a spark plug, from the point of view of weekend mechanic.

First let's cover the basic functions of the spark plug.

The spark plug has two primary functions:

1. Ignite air/fuel mixture

2. Transfer heat from the combustion chamber

Now that you know what a spark plug does, lets discuss how to remove the plugs.

This may be common sense, but please allow the engine and exhaust to cool down prior to performing this task.

1. With the engine cooled, disconnect the spark plug wire caps where they connect to the plug inside the cylinder head. My advice is to very gently pull of the wire caps. On one set I yanked at the wire cap and stripped out the inside. This was dumb, but it allowed me to buy a nice set of red wires.

2. With the spark plug wire caps removed, I suggest cleaning out the area around the plug. The best way to do this is with compressed air to blow out the debris and crud that has accumulated. Please leave the plug in for this, the idea is that you want to remove the crud so it does not fall into the cylinder head when you extract the plug.

3. Grab your spark plug socket, I forget the correct size for the Victory plug, and remove the plug. It does not take much to loosen the plug since they are only tightened with 12 lb-ft (Foot Pounds) of torque.

4. Now is the time to perform basic inspection of the plug. A plug is the best indicator on how well your engine is performing.

First check for the following:

a. worn out electrodes
b. check the insulators for cracks
c. and check the color of the insulator.

As mentioned, the spark plug insulator color gives you an indication of how well your freedom engine is operating.


- A light to light tan/gray color ... Condition is: GOOD. This light tan/gray color tells you that the spark plug is operating at optimum temperature and that the engine is in good condition.

- If the insulator is white .. Condition is LEAN and engine is running hot.

- When the insulator has a black sooty deposit or looks wet ... You have an Over-RICH condition and possible Oil control problems.

Installing the Spark Plugs

Whether you are replacing or installing a new spark plug, always check the spark plug cap with a gap or wire gauge. The gap specification is: 0.8mm (0.032in).

I run the NGK CPR6EA-9 spark plugs in my 204 Kingpin. 2003 Vegas bikes use model: NGK CR7EB

If you are disciplined, you will put anti-seize compound on the spark plug threads. I never have, but it is suggested.

The torque specifications for the plugs are: 16Nm or 12 lb-ft (Foot Pounds)

I hope this article has provided you with the basics on how to read the spark plugs from your motorcycle. Note that the same principle of reading the spark plug coloration applies to all engines, not just motorcycle engines.

Contributing author to Cycle Solutions http://www.cyclesolutions.net

and the Kingpin Cruisers http://www.kingpincruisers.net


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