All you need is the spark.
The spark plug on your dirt bike ignites the fire that starts the engine and provides a continuous spark thereafter firing one per full revolution on a 2-stroke and every other full revolution on a 4-stroke .
Fun Fact: A stroke is half a revolution hence the term 4-stroke.
We won't get into the ins and outs of how the spark plug works since that falls more in line with a section during motorcycle technician training. However, riders often forget about the spark plug or, on 4-strokes, procrastinate doing a visual check because of its location. Therefore, riders encounter a fouled spark plug, full of soot from oil or burned up, at the most inopportune time.
You can spend as little as $4 or as much as $40 (depending on make, model and year of dirt bike) on a spark plug but in either case spark plugs generally last quite a while, at least on 4-strokes. But keeping an eye on the spark plug prevents headache and heartbreak down the line. Whenever you ride and wherever you go, always carry an extra spark plug in your tool box. When riding trails, bring a spark plug, gap tool and spark plug removal tool with you to prevent getting stranded, especially if you expect drastic elevation changes. Spark plugs might be hard to reach but you can replace a dead one with a live one in relatively short order and get back to riding.
Mechanic's Tip: Elevation changes air density (temperature also plays a factor) so the higher you go the thinner the air meaning less air and more fuel and oil in the cylinder on a 2-stroke resulting in a fouled plug.
When to Change Spark Plug
4-Stroke Dirt Bike Spark Plug
On 4-stroke dirt bikes, check the service interval listed in the owner's manual. If you don't have the owner's manual or don't want to bother looking for it then change the spark plug in your 4-stroke dirt bike about once a year, if you mostly weekend ride. An engine rebuild offers a good time to replace the spark plug, too.
2-Stroke Dirt Bike Spark Plug
The oil mixed into the gasoline for 2-strokes often allows for the spark plug to foul out faster than 4-strokes, depending on carb tuning. Again, check the owner's manual but most 2-stroke riders tune their carb on the rich side which hastens along the fouling of the plug. If you ride a 2-stroke dirt bike, always have at least one spare spark plug in your tool box. Easy access to the spark plug on the cylinder head leaves no excuse to NOT routinely inspecting the spark plug prior to riding.
Elevation affects spark plugs on 2-strokes more than 4-strokes also because of tuning. You can't flip a switch, so to speak, on a non-fuel injected 2-stroke when riding different elevations therefore many riders stick with what works, or what worked last time. So, you ride a track day one weekend then head to the high desert for trail riding the next weekend without adjusting the carb for the elevation, expect the spark plug to foul faster. A fuel injected 2-stroke self-calibrates for elevation which helps prevent the plug from fouling.
Mechanic's Tip: Keep in mind, if you have a carb adjusted for higher elevation and ride near sea level you run lean and can burn a piston.
The spark plug on a 2-stroke can tell you a lot about how the bike is running. The electrode turns white when you run too lean (more air to fuel) or turns black and oily when running too rich (more fuel to air). Ideally, the electrode looks brown.
Spark Plug Number
Just as you cannot grab any pint of oil off the shelf to put in your dirt bike's engine you cannot grab the first spark plug you see and go. Not all spark plugs are the same and you don't need to spend money on a more expensive spark plug based on the idea of getting better performance. When choosing a spark plug first ensure it fits with the MMY of your dirt bike by consulting with the owner's manual.
Secondly, notice a numerical number in the type or model of the spark plug that looks like "B8ES" or "BKR7EIX" depending on manufacturer. That number is key to the correct spark plug. The higher the number the cooler the plug. You can get plugs as low as "5" or as high as "10." If you run a lower number spark plug in the spring you might want to swap it for a higher number spark plug during the summer, otherwise you might ruin the engine. The owner's manual for your dirt bike offers suggestions on the best spark plug number for the time of year and even geographical location.
Any other numbers or letters as part of the spark plug model refers to the plug's length, thread size and type of electrode. Different style electrodes can make a cleaner spark resulting in more complete fuel burn and other benefits.
Spark Plug Gap
The spark plug gap consists of the space between the center and side electrodes. If the gap is too big the plug won't fire or will miss fire, while too small of a gap causes the plug to ignite too soon missing the right time on the stroke to fire at compression. Incorrect gapping also causes unwanted higher engine temps. A correctly gapped spark plug fires at the right time and has optimal fuel burn per stroke. To determine the proper gap on your spark plug you need a spark plug gap tool and the bike owner's manual. The owner's manual tells you the appropriate gap (0.5mm, 0.6mm, 0.7mm and so on) for the MMY of your ride and the gap tool shows the plug's current gap. The gap can change if the spark plug was handled roughly during shipping or by you tossing the plug in your toolbox with little care. The gap tool serves as the fix-it allowing you to close or widen the gap, if necessary.
How To Change a Spark Plug
The spark plug sits inside the cylinder head and easily removes from the outside with a proper fitting socket wrench. A spark plug wrench works best because it provides the depth needed to fit the socket over the electrode and around the spark plug fitting to loosen it. Removing the spark plug on a 2-stroke dirt bike takes less than a minute. You normally have to remove the gas tank to get at the spark plug on a 4-stroke dirt bike.
How to Remove Spark Plug
Remove the spark plug wire by pulling off the cap. Remove the spark plug using the socket wrench. Replace with the new spark plug and be careful to line up the threads before tightening. If you strip the threads inside the cylinder head you have to replace the cylinder head. A more common problem comes from overtightening the plug which breaks off in the head. This normally won't ruin the cylinder head but have fun removing the broken plug. No easy feat! You can use a torque wrench to seat the spark plug but most owners go by feel, just don't overtighten.
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