Dirt Bike Fouling Spark Plugs - Causes & Fixes

Tired of having to replace the spark plug that keeps fouling on your dirt bike? It might be a simple and cheap fix, but there can be several variables to the issue.

In this article, you'll learn what it means when your 2 stroke or 4 stroke dirt bike engine fouls a spark plug, why it happens, and the best ways to fix and prevent it so that you can ride without worrying your dirt bike stalling and not starting.

What does it mean when my spark plug is fouled on my dirt bike?

A fouled spark plug is simply when the engine spark plug cannot ignite the air fuel mixture and stops the engine from running - generally caused by too much fuel.

This excess of fuel that cannot be burned leaves the spark plug looking black and sooty and/or oily.

Why is my dirt bike spark plug black?

Black on the spark plug is carbon built up on it. The carbon fouled spark plug is caused by the fuel that's not getting burned.

If an engine can completely ignite the air and fuel mixture, then there would be absolutely zero smoke or "emissions" to worry about. Unfortunately, gas engines aren't that efficient.

Smokes out or carbons up

So, the excess fuel either goes out as smoke, stays in the combustion chamber to get ignited on the next cycle, or causes black carbon deposits on your spark plug.

This carbon build-up is the black that sticks to the spark plug, top of the piston, and in the combustion chamber.

The carbon (fuel) eventually builds up on the spark plug so much that it can't ignite the air-fuel mixture: this is what's known as a fouled spark plug.

Can you clean a fouled spark plug?

You can try cleaning the black carbon off of the spark plug with some sandpaper, but it may not work. It might start and run, but it's a very temporary fix.

I would only recommend trying to clean a plug as a worst case scenario. For example: your plug fouls out in the woods miles from your truck and you forgot to bring a spare spark plug in your tool pack.

Dirt bike fouled spark plug symptoms

There are a number of signs that give you the heads up that your spark plug is fouling or is already fouled. You may have one or multiple of these issues, but if you feel or see one of these, you might be able to prevent it from completely fouling and not running.

These are the most common fouled plug symptoms for 2 stroke & 4 stroke dirt bikes:

Won't start

It can happen on a cold start, but you usually get the 'dirt bike not starting' symptom after the engine has been warmed up. When the plug is so wet or full of carbon, it won't even be able to spark and ignite the air-fuel mixture, causing your engine to not even start.

Won't stay running or idling

Maybe your can get it running once or a few times, but it just keeps dying. When your dirt bike won't idle, that's a good sign that the spark plug is fouling and needs to be replaced.
This could be over the course of hundreds of hours on a used spark plug, but it can happen quickly if there's too much fuel and/or oil getting into the engine.

Runs rough - sputtering

So you can get your 2 stroke or 4 stroke running, but when you give it gas it sputters - poor throttle response can also be a hesitation or bogging. This is another common sign that your plug is fouling, which means it should be replaced and then you need to figure out why it happened to prevent the same problem, which I'll cover shortly.

Excessive smoke

While it's more common on a 2 stroke, a 4 stroke dirt bike can also have a massive amount of smoke coming out the exhaust. When the spark plug is fouled or starting to, a common symptom is lots of white smoke out the exhaust because it can't burn all of the fuel efficiently.

Smelly or spooge out the exhaust

When you pay attention to what your dirt bike is saying, you'll see, hear, feel and smell new things when there's a new problem arising. One sign that your plug may be fouling is when the exhaust gets extra smelly or there's an oily spooge running down the exhaust muffler/silencer.

This is because the fuel mixture is too rich and you get more of a sooty gas smell out the exhaust.

Black spark plug

One of the most common signs of a plug fouling in your 2 stroke or 4 stroke dirt bike is a black spark plug. Now, this could just mean that your fuel mixture is too rich, but it's a small warning sign.

If you do a regular check of the spark plug and it shows up as black with carbon on it, there's a much higher chance that it will foul sooner than later.

Why does my spark plug keep fouling?

Now that you've learned the most common symptoms, it's time to cover what actually causes these issues in the first place. When troubleshooting a dirt bike problem, there are two tips that I always recommend to make your life easier and less frustrating.

Is it bad to let a 2 stroke idle?

Some people will tell you that it’s bad to let your 2 stroke dirt bike idle. Well I believe, based on my experience with owning and tuning 2 stroke bikes over the years, that it’s technically not bad and it won’t hurt your 2 stroke engine.

With that said, I understand why they say that, and it’s because the spark plug could foul and cause the engine to not start again.

Can a 2 stroke dirt bike idle without fouling a spark plug?

Simple answer: yes, your 2 stroke dirt bike can idle without fouling the plug. The problem is when you let it idle too long the more likely it will foul the spark plug.

So, can that be fixed? The main factor to how well your 2 stroke can idle is how well it is jetted. More on th

2 Pro Tips for troubleshooting your dirt bike problems

Number one is: start with the easiest things first. That means you shouldn't take the engine apart at the first sign of it not start. That example is a little extreme, but as you go through this list, just remember to try the quickest solutions first.

Number two: only do thing thing at a time. When you change multiple things, you won't know for sure what fixed the problem - or worse yet, you don't know what made the problem even bigger!

With that said, these are the most common reasons why your spark plug is fouled:

Incorrect jetting mixture

Jetting a 2 stroke is often a little more picky than a 4 stroke carb, but they can both cause a lot of problems when the jetting is off. When one or more of the jet circuits are too rich, they allow too much fuel into the engine, causing it to flood or give your 2 stroke a wet spark plug and foul.

A rich pilot jet circuit is the most common cause of plug fouling, and I can can show you how to easily tuning it in just a few minutes. For 2-strokes click here, or if you have a 4-stroke click here.

Not enough air = No Combustion

First thing to do if your spark plug has been fouled is to check the air filter. If it's really dirty, that makes it difficult for air to get through to the engine, causing a rich condition.

Either clean it thoroughly with some Air Filter Cleaner (Amazon), or buy a new filter.

Before you put it back in the air-box, treat with some air filter oil or spray. BE CAREFUL, though, because adding too much oil can also clog up the filter, resulting in another fouled plug.

Do you have the right 2 stroke oil mix ratio?

(For 2-Strokes) While the gas/oil ratio doesn't equate to what the jetting is, you can still end up with a fouled plug, or worse, if the mixture is wrong. Depending on your bike, it's best to look at your owner's manual for the correct pre-mix ratio.

For most 2-stroke motocross bikes, a good ratio is 32:1. Gas is 32 times more than the amount of oil.

For example, it means that for every 128 ounces (one gallon) of gas, you add 4 ounces of 2-stroke oil for a 32:1 ratio, or just over 3 ounces to a gallon for a 40:1 ratio.

If you have too much oil in the mixture (such as 12:1), then you will probably end up fouling the plug because it is more difficult to burn that much oil. Don't try to cut corners with the oil though, because it's very important for your dirt bike's engine.

Not enough 2 stroke oil? Toss the grenade...

2-stokes need the oil for lubrication on the cylinder walls, so if you don't have enough (or any for that matter) then the cylinder walls will run dry, causing it to overheat and seize the engine very quickly.

So it's very important that you put in the right mixture pre-mix to your 2-stroke gas tank. Or keep enough oil in the tank for an oil-injection system.
To understand the science and importance of pre-mix read more here.

Cracked reeds

Also, 2-strokes have reeds that air and fuel go through into the cylinder, so if they are cracked or broken the bike will not run right.

This could cause the plug to foul, so before you go out and buy anything expensive, make sure the reeds are still good.

Plug temp is too cold

While this usually isn't the main problem, it could be that your spark plug isn't burning hot enough. Having a hotter spark will ignite the fuel stronger, leaving less residual gas/oil in the cylinder that could cause the plug to foul.

This isn't the best choice long-term, but I'd say it's safe to go one plug hotter than stock. Spark plug manufacturers aren't always the same, so make sure you find out how they rate them.

What's the right spark plug for a 125 2-stroke?

For an example, an NGK BR9EG is commonly used in a 125 2-stroke dirt bike. One plug hotter would be a BR8EG.

I have done this before while out trail riding for a weekend since I didn't have time to re-jet the carburetor.

Compression is too low

When your engine compression is too low, the air and fuel mixture can't be efficiently compressed and burned. This leaves extra fuel in the engine and causes your spark plug too foul.

Can a 4 stroke engine foul a plug?

Don't be fooled into thinking that only 2-strokes foul plugs. 4-Strokes, although it is not as common, do foul plugs.

Like I mentioned before, improper jetting is the main reason why dirt bikes foul spark plugs.

Many motocross bike comes rich from the factory (especially Honda 2-Strokes when they were being made). It's usually a pretty simple fix, but many riders are too lazy and would rather continue buying plugs instead of spending a few bucks on jets.

Weak Spark Can't Burn the Mixture

Whether it's a loose spark plug, cap, bad wire, or faulty stator, you need enough juice to ignite and burn the air fuel mixture.

How to stop spark plugs from fouling

Now you know what causes a dirty spark plug or wet fouling, so lets get to fixing and preventing the problem from happening again in your 2 stroke dirt bike or 4 stroke if you're having troubles.

These are the best ways to fix and stop spark plugs from fouling:

Fix the jetting

If you know for sure it's a jetting issue that's causing the fouling, then you'll want to lean out the air/fuel screw and possibly go down on the pilot jet. This is especially important if you ride in the lower RPM range often.

Your goal is to get a crisp throttle response at any rpm. You may also need to go leaner on the clip position.

For high RPM riding and it's still sputtering or fouling plugs, then the first thing you should do is go down a size or two on the main jet. The main jet controls roughly ½-full throttle position openings.

By going leaner on your dirt bike's carb, it will not foul plugs as easily, allowing you to ride all day long. If it's properly jetted, you should never foul a spark plug, assuming your engine is in good running condition.

Just ride harder

If you want to go the easy route (well not really!), just ride the bike harder. Spark plugs usually foul at low RPM's, so if you're wringing your bike out you shouldn't have as much of a problem!

Okay, so this isn't a longterm actual fix, but it can help stop your spark plug from fouling if you have a long ride back to your truck. Just ride it in a lower gear and keep the RPM up higher when it's safe to do so. It helps if you have confidence in your riding skills.

Time to rebuild if nothing else works

If you've tried all of the above, then there's a chance that your top end may need to be rebuilt. A worn out piston can cause a rich condition, which results in the plug fouling.

Not only can this solve your plug/jetting issues, but it may prevent a major engine failure in the near future because of worn out parts.

Most common causes

While there are many causes of a fouled plug in your dirt bike, it usually doesn't take long to narrow down the probable root cause.
The most common reasons why your dirt bike has a fouled spark plug are:

• Incorrect carb jetting
• Not enough air getting to the carb
• Improper pre-mix ratio
• Worn reeds
• Plug temp too cold
• Engine has low compression
• A weak spark

How to prevent fouled plugs with proper jetting in minutes

Do you have a 2 stroke that keeps fouling plugs? The air screw and pilot jet are the most important jetting circuits when it comes to fouling plugs. Want to learn how to easily tune the air screw to prevent it? Tap here to learn how.