5 steps for easy spark plug maintenance

How to help your engine run longer and burn cleaner

By Andrew LuptonAndrew Lupton


Photo by Eden Robbins

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Only a few inches long and a few dollars each, these workhorses pulse dozens of times every second in a small engine’s combustion chamber.

Though humble enough, the simple spark plug can offer unrivalled insight into the inner workings of your small engine, and routine maintenance helps your engine run longer and burn cleaner. Here’s what to do:

1. Remove

Slip the wire off the spark plug’s terminal end, clean off any loose dirt, and back the plug out using a special socket for this purpose. Don’t touch the plug while the engine parts are turning (even if the engine isn’t running), or you could get a nasty shock. Also, always disconnect the spark plug wire before starting any kind of engine work to prevent an accidental start.

2. Inspect

Take a close look at the electrode—the end of the plug that resides inside the engine. Brown and greyish-tan deposits are normal. Black goop means oil is getting past the cylinder and into the combustion chamber. Your engine may need new piston rings, a valve job, or both. A blistered, white electrode suggests your engine is running hot. This could be caused by insufficient cooling (look for dirt caked on the engine-block fins and check the oil level) or an air-fuel mix that is too lean (too much air, not enough fuel), suggesting a carburetor problem. Dry, sooty deposits mean the mixture is too rich (too much gas, not enough air). Clean your air filter and make sure the holes in the gas cap aren’t clogged. If the electrode is hammered down, the wrong plug—one with an overlong reach—has been installed and the electrode is being smashed flat by the piston.

3. Test

If your engine won’t start, use a spark tester to check the ignition system. There are two common styles of tester: one that clips to the spark plug wire and the engine block and another that cradles the wire in a metal sleeve. With the tester in place, pull the starter cord. If no sparks appear, your ignition system is faulty. If you see sparks, the plug could be the problem. Try a new one.

4. Set the gap

The plug’s electrode gap must be set to an exact distance for the engine to fire properly. The owner’s manual will specify the correct gap for your engine. To measure, use a small feeler gauge (available at automotive stores), which has wires or blades of varying thickness. Select the one you want and place it inside the gap; it should slide in and out with only a bit of resistance. Most gauges come with a wrench-like attachment that you can use to adjust the gap by gently bending the electrode arm.

5. Replace?

Plugs are so cheap, many cottagers buy new ones as part of a regular tune-up. Just remember to check and, if necessary, reset the gap before installing. There are scores of different plugs; get the one called for in the owner’s manual. Carefully hand-thread the plug a few turns before reaching for the socket wrench: Cross-threading will mean that, in a best-case scenario, you’ll need new threads tapped into the hole you just wrecked. Worst case? A new engine head.

This article was originally published on January 24, 2011

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Andrew Lupton