4 Signs That Your Motorcycle Spark Plugs Are Bad


If you’ve owned a motorcycle for more than a few years, you’ve probably run into a few issues with the sparks plugs. This is especially true if you own an older motorcycle.

Spark plugs are small, but they provide a vital component to the functionality of the engine. If there’s any part of them that is malfunctioning, that could mean your whole motorcycle acts up and gives you problems.

What are the signs of bad motorcycle spark plugs? The signs of bad motorcycle spark plugs include your motorcycle misfiring, backfiring, or frequently having a flooded engine. You can also look for physical signs on the spark plugs themselves which may include burn marks, broken tips, rust, and corrosion.

Throughout my motorcycle riding years, I’ve replaced hundreds of spark plugs mostly because the motorcycles I restored required it. I’ve been able to learn a thing or two about them through my experience as a motorcyclist and can report what I’ve learned.

Signs Of Bad Motorcycle Spark Plugs

Diagnosing bad spark plugs on a motorcycle can be difficult because the symptoms they manifest can often be associated with other issues. Knowing the symptoms of bad spark plugs can help you be better prepared in the future and help you fix your motorcycle much more quickly.

Each motorcycle is different with the amount of spark plugs they have depending on how many cylinders are inside the engine. The bigger the engine your motorcycle has, the more likely you’ll run into spark plug issues because you’re dealing with more of them.

The first common sign that your motorcycle spark plugs are going bad is if your motorcycle is misfiring. Misfiring is when your motorcycle is running on a steady rhythm and you can obviously tell it goes off of rhythm for a few seconds then catches itself back up. Think of it like the bass player in a band; you can obviously tell they’re off because the rest of the music doesn’t sound right.

Misfiring is caused by the spark plug not arcing at the tip at the right time. If it doesn’t arc and give off electricity at the right time, the gas it’s supposed to mix with isn’t where it should be and combustion is then thrown off.

The second way you can tell your motorcycle spark plugs are going bad is if it’s backfiring. Backfiring is when you hear random loud pops coming out of the exhaust. This is caused when the spark plugs has an intermittent spark.

An intermittent spark is when the spark plug usually fires just fine most of the time, but every once in a while it just won’t spark and your engine will backfire.  When there is fuel and air in your cylinder and the spark plug doesn’t ignite it, it gets pushed out of the cylinder on the compression stroke and as soon as it hits the hot exhaust header it combusts. See my article here for other reasons why a motorcycle backfires.

A flooded engine could also be a sign of one or several bad spark plugs. A flooded engine happens when you try to start your motorcycle and cranks over and over without actually starting. This will often give off a strong smell of gas.

When a motorcycle engine becomes flooded, that means the combustion chamber is filling with gas but isn’t being combusted. The carburetor or fuel injector continue to give the appropriate air and fuel ratio to a dysfunctional spark plug that isn’t arcing and isn’t creating the combustion needed to move the pistons up and down. Click here to see my article about what to do when your motorcycle engine floods.

If your exhaust smells strongly of gas or if you notice a little bit of gas spraying out of the exhaust pipes, your motorcycle may have bad spark plugs. If the spark plug is bad and isn’t giving a spark at the right time, left over gas in the combustion chamber gets sucked into the engine, doesn’t combust, gets past the exhaust valve, and shoots out the exhaust pipe.

If you do notice some sort of liquid coming out of your exhaust pipe, make sure it isn’t water. Sometimes condensation can build up in there and shoot out when the motorcycle is first started.

Another sign you can look for if you suspect bad spark plugs is looking at the condition of the spark plugs themselves. There are obvious clues that will show that the spark plug is bad such as any burn marks, if it’s completely white, if any parts of the nobs are broken or bent, or if there is any rust or corrosion at all.

Any of these ailments on a spark plug can cause the spark plug to delay it’s electric arc or simply not spark at all, especially if there’s any type of corrosion on the end.

What Causes Spark Plugs To Go Bad

Once you have figured out that bad spark plugs are the problem, you’ll probably want to know how to prevent this problem in the future. Don’t feel too bad if you end up having to replace your spark plugs because the number one reason they go bad is due to age.

I wish we could say that all spark plugs last forever (as well as any replacement part you buy!). Over time, the nobs on the end of the spark plug simply wear out from sparking. Anything that has to do with the electric components on a vehicle will likely go out sooner than other parts (even when everything is taken care of properly).

Bad timing on your engine can wear out the spark plugs much faster than their expected life span. If your gas is detonating too early or too late inside the combustion chamber of the engine, all of that back pressure is going to build up on the spark plug since the pressure doesn’t have anywhere else to go.

A motorcycle running lean can also quickly wear out the spark plugs. This means the carburetor or fuel injector is not inserting enough gas inside the combustion chamber. The gas that does get inserted burns much hotter because there’s more air than there should be. That hotter temperature can easily warp and bend spark plugs and even ruin them within a day.

How Often You Should Change Your Spark Plugs

I am honestly relieved when I find that bad spark plugs are the reason my motorcycle is acting up. Spark plugs are an easy and cheap fix and can be done within an hour.

You can ride your motorcycle for years until you notice it’s acting up due to bad spark plugs, but you should probably take a look at them before they start giving you trouble. Motorcycle spark plugs should be replaced every five years. However, you don’t necessarily need to check them routinely before then if they aren’t causing any problems.

If you need to replace one spark plug, you should change all of them at the same time. Again, spark plugs are inexpensive and easy to change so you might as well change all of them while you’re at it. If one spark plug is going out, chances are the others will soon follow.

How To Replace Bad Spark Plugs

Replacing spark plugs only requires a few simple tools, a few dollars, and about an hour of your time. If you’re changing out your spark plugs, the first thing you’ll need to do is disconnect the battery to remove any risk of shock.

The next thing you’ll need to do is remove all the spark plug boots off of the spark plugs. Those should easily come off with a little bit of a tug. At this point the back end of the spark plug is exposed. The ends are shaped like a nut so you can use a socket wrench to get it out. Most spark plugs measure 5/8″.

There are special sockets out there that have rubber gaskets inside, so when you finish unthreading the spark plug the socket will be able to pull it out for you. Sometimes spark plugs can be hard to get to so a tool like this is very handy.

A lot of people assume that when they buy new spark plugs, the gap at the tip of it is already set at the right distance. This actually isn’t true and you need to set the correct gap yourself.

There is a tool that looks like a silver dollar that has a ridge that goes from skinny to wide as it goes around the edge (pictured above). You can easily use this tool to set the right gap at the tip of your spark plugs. You’ll need to look in your owner’s manual or look online to see what gap you should have with the spark plugs specific to your motorcycle.

Once you set the appropriate gap on your spark plug, you’ll need to coat the outside of the spark plug with anti-seize. Most motorcycle engine blocks are aluminum, so when you thread a steal spark plug into an engine block the two metals want to seize together because they’re different from each other. The anti-seize solution will prevent that from happening.

Now you can thread in all of your new spark plugs into the engine with your socket wrench. Replace the spark plug boots and hook the battery back up to the motorcycle.

Related Questions

How do I lessen the gap on a spark plug? If the gap is too big on a spark plug, lightly pushing down on it with your hand may do the trick. If not, you can try pushing it down by gently pressing the side electrode on a hard surface. Make sure to check the gap once you close it a little to ensure it’s at the right gap distance.

How often should I get my motorcycle serviced? The oil on a motorcycle should be changed every 4,000 miles or every six months (whichever comes first). Lubing and checking the tension on the chain should follow the same intervals. The tires should be check every month and tuning the carburetor and flushing the cooling system should be done every two years. See my article here for more information.

Recent Content