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How to: Maintaining a Small Engine Spark Plug

A spark plug is essential to the operation of a gasoline engine. It simply transfers electric current from an ignition system to the combustion chamber, of an engine, to ignite the compressed fuel/ air mixture. A clean spark plug allows for easier starting and efficient engine performance. A dirty, corroded, or incorrectly gapped plug does just the contrary. Correctly determining the condition of a spark plug can be a useful indicator in evaluating an engine’s overall operating status. For this article we will be inspecting the spark plug of a Troy-Bilt push mower powered by a Briggs and Stratton engine. A typical engine spark plug. (Photo by Champion spark Plugs)
Above: A typical engine spark plug. (Photo by Champion spark Plugs), Below: Author's Troy-Bilt Push Mower

Author's Typical Troy-Bilt Lawnmower

1.) To begin we must first locate the spark plug on the engine. Most spark plugs are located on the cylinder head with a single ignition cable connected.
Found the Spark Plug Location

2.) Grip the ignition cable closest to the spark plug and “wiggle off“. Be careful to not yank or pull as this may cause damage to the cable. (Some ignition cables may require a careful twisting motion to remove.)
Removing Spark Plug Wire

3.) Before continuing clean the area around the spark plug to prevent foreign particles from entering the engine. (Failure to do so may result in premature engine wear!) For this application we will be using a ratchet with a 5/8” deep socket. If the plug does not unloosen easily, re-connect the ignition cable (see step 9), run the engine to operating temperature, and then shut off. Wear heat-resistant gloves and try unloosening again. Be advised that the engine is now hot including the plug!
Removing the Spark Plug.

4.) Now with the spark plug removed we can inspect for any deficiencies and determine the engine’s overall operating status. Carefully inspect the plug for heavy deposits, cracks, corroded electrodes, or damaged threads. Replace if necessary with the correct spark plug for your engine model.
Here's the Spark Plug after removal

5.) Upon closer inspection we can evaluate the engine’s overall operating condition as: normal with a slight lean condition. Carefully clean the spark plug with a wire brush and if necessary use a sturdy knife to remove hard deposits. (Do not use power tools to clean.)
Evaluating the Spark Plug - Looks Clean
Example Inspection Chart by Denso
(Top Photo by Author, Lower Photo by Denso Spark Plugs)

6.) It is important to verify the plug’s electrode gap for optimal performance. Too large of a gap and the plug will have difficulty firing, but too narrow of a gap will result in a small, weak spark. Specifications for this engine call for 0.020 in gap. Refer to your engine’s manual or a repair manual that covers your engine model. If you can't locate your engine’s spark plug gap try 0.030 in. Checking the gap simply involves a feeler gauge of appropriate size. This spark plug has the correct gap due to slight resistance felt when inserting.
Editor's note: While a flat feeler gauge is shown in use, if you have well worn (rounded) plug electrodes, better to use a wire type spark plug gap gauge, for accuracy.. Even better, if plug is bad enough to have electrode tips rounded, replace it with a new one. Clean,"sharp" spark plug electrode edges improve ignition performance
Look up Tune-Up Specifications in Owner's Manual
Gapping a Spark Plug with Feeler Gauge

7.) If adjustment to the gap is necessary use the hook part of the feeler gauge for widening the gap. Slightly tap the plug’s side electrode on soft wood to close the gap. Editor's Note: Most spark plug gap tools can use the gapper hook to close gap, too. When using the feeler gauge hook gently tug on the side electrode ( think fulcrum) being careful to not over bend.
Gap Adjustment with Gapping Tool

8.) We have successfully removed the engine’s spark plug, inspected for deficiencies, evaluated the engine’s overall running condition, and checked for proper gap clearance. We are now ready to re-install the spark plug. Gently screw the plug back finger-tight while avoiding to cross the threads. Specifications for this engine model call for a 180 lb-in torque value. A 5/8” deep socket with a quality torque wrench will be used. (Be careful to not over torque as this may crack the plug or strip the threads!) Editor's Tip: IF a torque wrench is not available: If you're installing a NEW spark plug, with gasketed seat (like the one pictured) thread it in to just snug (Finger Tight when using fingers to turn socket/extension) and then tighten 3/4 of a turn (NO MORE THAN THAT) should be pretty close. With a USED spark plug, it's just under 1/4 turn to tighten. but again, if you have a torque wrench, *use it*
Installing the Spark Plug
It is always best to use an accurate Torque Wrench

9.) After the spark plug has been correctly torqued we are now ready to re-connect the ignition cable. Simply push the cable lead onto the plug ensuring a good fit. Verify the correct engine oil level and run the engine to normal operating temperature.
Re-Connect Spark Plug Wire

Congratulations you have successfully removed, inspected, and installed your engine’s spark plug. You now know how simple and easy it is to evaluate the plug’s condition and check the electrode gap for optimal performance. These basic steps may be applied to many engine types.

Author's References:

Briggs & Stratton. Service and Repair Instructions. Milwaukee: Briggs & Stratton, 1984. (Print.)

Dempsey, Paul. How to Troubleshoot & Repair Any Small Gas Engine. 1st ed. Blue Ridge Summit: TAB, 1985. Print.

Champion Spark Plug. Digital image. Champion Automotive Spark Plugs. N.p., n.d. Web. 8 Nov. 2013. http://www.championsparkplugs.com/Champion-Automotive-l1343.aspx.

DENSO Spark Plugs. Digital image. Global DENSO. N.p., n.d. Web. 8 Nov. 2013. http://www.globaldenso.com/en/products/aftermarket/plug/.

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22.11.2013. 23:17 | by Jeff Moore


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