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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.

Incorrect jetting is one of the most common causes of plug fouling, but we’ll look at what the top reasons why your dirt bike keeps fouling spark plugs and how you can fix it before spending a ton of cash on continually replacing spark plugs.

Why is my dirt bike spark plug black?

Black on the spark plug is carbon built up on it, and it’s 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.

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

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

Rich jetting causes spark plug to foul
Turn Up The Heat!

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.

The most common reasons why your dirt bike has a fouled spark plug are because a dirty air filter, improper pre-mix ratio, carb jetting is too rich, the engine has low compression, or a weak spark.

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.

No 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.

Dirty Air Filter causes spark plug to foul
A dirty air filter is bad for many reasons

What Mixture Are You Smoking?

(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 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. To understand the science and importance of pre-mix read more here.

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.

Hotter Plug to Resist Spark Plug Fouling?

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.

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.

Can a 4 stroke 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.

What to do if getting causes plug fouling

If you know for sure it’s a jetting issue that’s causing the fouling, then the first thing you should do is go down a size or two on the main jet.

Your goal is to get a crisp throttle response at any rpm. If you ride in the lower rpm range often, then you’ll want to go down on the pilot jet and then possibly go leaner on the clip position (learn the Basics of Jetting Here).

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.

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.

You Can Always 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!

If Nothing Else Works!!!

If you’ve tried all of the above, then there’s a chance that your top-end may be lacking in compression and needing attention. 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.

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.

Ken

Sunday 28th of February 2021

I also suggest checking that the power valve is working. I had a lot of fouled plugs at low rpm because my power valve was stuck open

Preston

Sunday 21st of March 2021

I just bought a brand new 2021 Yz 250 from the showroom and I went out for a ride 3 hours everything was fine .. next day took my bike out and it wouldn’t kick over..pulled the plug and put a new one in .. rode for about 3 hours again and the bike died on me..took the plug out and it was black and had a strong smell of gas on it. Could it be the jetting cause I went to another area? Only went 100km south of where I bought the bike

Kelley Fager

Monday 1st of March 2021

I agree, thanks for the tip, Ken!

Dalton

Thursday 31st of December 2020

I have a 2019 kx65 with a Lectron carb. We bought the bike brand new when it just came out and put the carb on then. It ran good for a couple months then started fauling plug after plug. We have done a new top end. Might have 10 hrs on it by now. Bike gets ran hard in MX races and also AX races. At a loss on what could be the problem. Any ideas on what could be causing this problem?

Kelley Fager

Thursday 31st of December 2020

Hey Dalton, sorry to hear about your problem. Have you tested the compression? I haven't personally used a Lectron, but I'm somewhat familiar with them. Did you tune it when you got it? Have you taken it off to inspect for any damage? Does the bike run clean and crisp at all throttle positions?

Kadin

Monday 21st of December 2020

I have a 1996 Cr 250 with a full rebuild as of 6.7 hrs ago. I run 32:1 mixture and got a new spark plug and it still will not start. But it will bump start and run good for the next couple days before it needs another bump. Any suggestions on what to do next?

Kelley Fager

Tuesday 22nd of December 2020

Hey Kadin, is the spark plug repeatedly fouling? Was it doing this before the rebuild? Have you tested the compression?

La6

Wednesday 27th of March 2019

Other way

Jace backer

Sunday 3rd of March 2019

So I bought a 1997 Kawasaki 250cc dirt bike a couple of years ago and it ran well at first then it started having problems with the spark plug it would die about every 20 minutes when I would ride and it wouldn’t start unless I put a knew one in and when I’d put a new spark plug in it would start right up so I took it to a shop and he put in a new piston and did a bunch more stuff to it and when I got it back it would still go through spark plugs and when it would get hot it would start to bog out so I asked the guy that was working on it if he knew what was wrong and he said he had no clue and he tried everything but he couldn’t fine what it was so I don’t know what to do

Kelley Fager

Thursday 2nd of January 2020

What kind of shape is the cylinder in? If the cylinder is scored then just putting in a new piston won't solve your issue. Hard to say over just that information though.