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Find the right parts for yourHarley-Davidson
Old spark plugs and wires can make a huge difference in the way your bike runs. They are cheap and easy to replace, too! Why wouldn't you take advantage of that?
This article applies to all Dyna Glide models (1991-2016). Download fullmetal alchemist 1-64.
Getting spark is essential to making power reliably. New plugs and wires are a great start to upgrades on your motorcycle. Some people, like myself, change their plugs almost as often as they change oil. While that isn't necessarily required, a pair of plugs are cheaper than a quart of engine oil, and they really can make a difference! Keep reading and I'll walk you through it.
Materials Needed
- 5/8' spark plug socket and ratchet
- Dielectric grease
- Spark plugs (I prefer NGK)
- Spark plug wires (I prefer ACCEL for all my vehicles)
- Wire brush (optional)
Step 1 – Ensure that the ignition is off
Verify that the bike's ignition is off completely. Make sure that the key is in the 'OFF' position. You could actually disconnect the battery here for further safety and reassurance if you were so inclined, but it is not necessary.
Pro Tip
If your ignition switch is not clearly marked, simply twist the key as far counterclockwise as it will go and remove the key. Do not use too much force when turning your key!
Step 2 – Disconnect the old spark plug wires
They will just pull off of the ignition module/coil and the spark plugs. Make sure to pull them directly off of the connections by gripping the thicker boots, not the thinner wire itself. If you are replacing the wires with a new set, you can discard these. Pay particular attention to what ends of the wires are connected to the plugs and which ends are attached the the ignition module. If you need help remembering, you can take a picture of the old wires on the bike with your cell phone before you start.
Pro Tip
Check the connections inside the old wires for corrosion. If you are reusing them, be sure to clean them well with a wire brush.
Step 3 – Remove old spark plugs from the cylinders
Brush any dirt or debris off the motor in the area around the spark plugs to avoid getting it in the cylinder when you remove the plugs. Using a 5/8' deep well socket, carefully remove the plugs from their place in the cylinders' heads.
Pro Tip
Visually inspect your old spark plugs before discarding them. The color of the tip of the plug can indicate how well your engine is running.
Step 4 – Install new spark plugs and wires
Place your new plugs in the cylinders and tighten them gently by hand. Starting the plugs by hand helps reduce the risk of cross-threading the holes. Spark plugs should not be over-tightened; the rule of thumb is to snug them up little more than finger tight, then add 1/4 of a turn to crush the gasket ring. Any more than that, and you run the risk of cracking the cylinder or breaking off the plug. Using dielectric grease, cover the conductor at the end of the spark plug where the wires connect as well as the connectors on the ignition module. Connect the spark plug wires oriented in the same way the old wires were connected. Wipe off any dielectric grease that squeezed out when the wires slid on.
Pro Tip
Hand-tightened plus 1/4 turn is what most people use, but if you would like to use a torque wrench, you can. The torque value for spark plugs is 12 to 18 ft/lbs.
Related Discussions and Site
- Plug Reading 101 - HDForums.com
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- How to Read Spark Plugs - OnAllCylinders.com
Your spark plugs and spark plug wires, or high tension (HT) leads, are at the center of proper engine performance. Failing to change them when needed can negatively affect your riding experience. All the fancy intake and exhaust tuning in the world won't make a difference if the fuel mixture is not efficiently burned. This is a very straightforward job you can do yourself, even with limited mechanical experience. In this article, we'll show you how to change the spark plugs and HT leads with basic tools at home.
This article applies to the Harley Davidson Sportster.
Spark plugs are simple, inexpensive components of your ignition system and, on the Sportster, very accessible, so there's no reason not to keep them fresh. A good plan is to replace them every 8,500 miles or so, or roughly every other oil change. The spark plugs and HT wires work together with the ignition coil and other components to produce a spark strong enough to leap across the gap at the electrodes of the plug. When installed in the engine, the electrodes are located in the combustion chamber and deliver a properly timed spark to ignite the fuel/air mixture provided by the fuel system. If the electrodes have worn to the point where the gap becomes too large for the spark to jump or the HT wires have become old and electrically leaky, a misfire can occur, resulting in poor running, stalling, excess emissions and even an inability to start the engine at all.
Materials Needed
- 13/16' spark plug socket or box end wrench
- Spark plug gap setting gauge
- Anti-seize compound
- Dielectric grease
- Torque wrench
Step 1 – Remove spark plug wire from spark plug
We'll do the front cylinder first. Position yourself on the left side of the bike in order to see the spark plugs and wires. Grasp the spark plug wire by the cap, not the wire. There may be a metal shield present on some models; if so, remove that shield first.
Pro Tip
Be careful not to rock the cap back and forth too vigorously, lest you risk breaking the spark plug or cap.
Step 2 – Remove spark plug
Using your 13/16' spark plug socket, carefully remove the plug by rotating it counterclockwise. The engine should be cool to avoid damage to the threads and possible burns to your hands.
Don't force the plug to turn; if it seems too tight, try spraying the base of the plug with some penetrating oil.
Pro Tip
If you use a purpose built spark plug socket, there will be a foam cuff inside to retain the plug once it is free of the engine.
Step 3 – Repeat to remove plug from rear cylinder
Repeat the same steps used above to remove the rear spark plug. Once both plugs are removed, they can tell a story about how your engine is running. It's a good idea to examine them for any trouble. Compare them to the chart below.
Step 4 – Remove spark plug wires
Now, let's replace the spark plug wires or HT leads. We'll do one at a time. When you purchase new leads, the longer one goes to the rear cylinder and the shorter one to the front cylinder. The leads connect to the coils, which on the Sportster are located just above the front cylinder head behind the steering neck. In some cases, the coil set may be located between the 'V' of the engine on the left side. You may need to remove a cover or slightly lift the fuel tank to ease access to the coil. The spark plug lead presses directly into the coil. Remove the old lead by pulling straight out on the boot, not the wire. Match the length of the new wire and press the coil end of the wire into place until you feel it snap securely onto the coil. Route the new lead exactly the same way the old one was routed.
Now, repeat the process for the other cylinder. Once the wires are installed and properly routed, you are ready to install the new spark plugs.
Pro Tip
Before removing the old HT leads, carefully note how the wire is routed. A digital camera can be handy here.
Step 5 – Gap new spark plugs
![Harley evo spark plugs gap Harley evo spark plugs gap](/uploads/1/2/3/7/123711747/525379755.jpg)
Before installing the new plugs, you'll want to prepare the plugs first. Check the electrode gap with a spark plug gap gauge. Check the proper gap specification for your year and model bike, but it should be around 0.035' to 0.040'. Most plugs come pre-gapped with a plastic or cardboard tube to protect the side electrode in shipping, but if the gap needs to be adjusted, use the gap gauge to gently bend the side electrode until the correct gap is achieved
![Harley Evo Spark Plugs Harley Evo Spark Plugs](/uploads/1/2/3/7/123711747/622793050.jpg)
Pro Tip
Avoid levering against the center electrode when adjusting the the gap. The center electrode on some plugs can be extremely fragile and it's possible to crack the porcelain insulator nose. Damage to either will ruin the spark plug.
Step 6 – Install new spark plugs
Before installing the new plugs, apply a small amount of anti-seize compound to the threads of the plugs. Anti-seize will prevent corrosion of the threads and make removing the plug easier the next time. When installing new plugs, always start them into the thread and turn them as far as possible by hand. In most cases, the plugs will screw all the way down to their seats easily. This insures that you don't cross thread the spark plug hole, which will be an expensive proposition to repair.
Step 7 – Tighten new spark plugs
You'll want to make sure the plugs are properly tightened, but it does not require much. Ideally, a torque wrench should be used to tighten the plugs to about 14 to 16 ft/lb. If you do not have a torque wrench available, run the plugs down until they seat by hand and then tighten only 1/4 turn from there. All that you need to do is slightly crush the seal washer on the plug.
Step 8 – Reattach spark plug wires to plugs
In this step we'll re-attach the spark plug wires. Before you do so, use a little dielectric grease in the insides of the spark plug caps. This will prevent the caps from getting more or less 'welded' to the spark plugs from the heat of the engine. An additional benefit is that it may make the plug caps a bit easier to seat back onto the plugs.
At this point you are finished. Replace any covers or shields you may have removed for access and you're ready to ride!
Featured Video: Sportster Oil Change and Spark Plugs
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The ignition coil on a Harley Davidson motorcycle is an essential component of the ignition system. It consists generally of two sets of wires that are wrapped around an iron core. This component comes sealed, and is not a repairable item. Fortunately, testing is relatively straight forward, requiring a pair of resistance measurements made at the connection terminals on the coil. The time necessary to complete this task should be no more than 15 to 20 minutes.
Step 1
Turn off the motorcycle. Allow the engine to become cool to the touch before proceeding.
Step 2
Disconnect, by hand, the two spark plug wires from the coil. Unscrew the smaller connections on the coil with an SAE wrench.
Step 3
Set the ohmmeter to read resistance on the 'Rx1' setting. Measure the resistance across the two smaller terminals on the coil. The meter should read between 2.5 and 3.1 ohms.
Step 4
Set the ohmmeter to read resistance on the 'Rx1000' setting. Measure the resistance across the two larger terminals on the coil. The ohmmeter should read between 10,000 and 12,500 ohms.
If the meter reads outside of the ranges for either of the above measurements, then the coil is faulty and should be replaced.
Tip
- Sometimes a coil will only give a faulty reading when it is hot from riding. In this instance, it may be necessary to allow the coil to heat up before performing a resistance measurement. Exercise extreme caution when working around a hot motorcycle.
- 'Service Manual for Softail Models;' Harley-Davidson; 2000
- motorcycle image by Greg Pickens from Fotolia.com