Why Does My Dirt Bike Keep Fouling Spark Plugs?

Sick of having to replace the spark plug on your dirt bike almost every ride? Chances are, it’s a fairly easy fix. But there can be several variables to the equation. Incorrect jetting is the number one cause of plug fouling, but we’ll get a little deeper into what the most common causes could be and how you can fix them before spending a ton of cash on spark plugs.

No Air = No Combustion

Dirty Air Filter

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, 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.

What Mixture Are You Smoking?

(For 2-Strokes) While the gas/oil ratio doesn’t equate to what the jetting is, you can still foul plugs, or worse, if the mixture is wrong. Depending on your bike, it’s best to look at your owner’s manual for the correct ratio. For most 2-stroke motocross bikes, a ratio is 32:1 or 40:1, gas being 32 or 40, and oil being 1. For those that don’t it, it means that for every 128 ounces (one gallon) of gas, you add 4 ounces of 2-stroke oil for a 32:1 ration, or just over 3 ounces 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. 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.

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.

Too Rich

Turn Up The Heat!

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, 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.

4-Strokes Foul Plugs Too…

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. Almost every 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 a jets.

Can 4-Strokes Foul Plugs?

If you know for sure it’s a jetting issue that’s causing the fouling (you’ll know this by looking at the plug, which I’ll cover on how to read one in a future article), then the first thing you should do is go down a size or too 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 lean out the clip position (learn the Basics of Jetting Here). By leaning out your bike, your bike will not foul plugs as easily, allowing you to ride all day long.

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 your dirt bike will need an engine rebuild. A worn out piston can cause a rich condition, which results in the plug fouling. So if the top-end hasn’t been rebuilt for a while, you’ll want to do that. 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.

Good luck, and keep that bike running strong!

-Tom Stark

Wikipedia: Revolutions per minute is a measure of the frequency of a rotation.

58 thoughts on “Why Does My Dirt Bike Keep Fouling Spark Plugs?”

  1. Hi ya Tom, I have a blog (on which I use the name `cars2scrap’) and I liked this post so much that I have advised my readers to take a look. (Take a look at my most recent article.) I just wanted to thank you for taking the time to post it! – cars2scrap.

  2. I have a 2002 yamaha yz 125 2 stroke and I keep getting heavy oil deposits on the plug in the space of minutes. It won’t start and causing hesitation and cutting out alot. I checked the air filter which is fine and also the fuel mixture is good. The bike not running well at all. I’m really not sure what the proble is? Will it be the priston rings? if so how much would this cost to fix.

    Thanks

    Gary

    1. There could be a few different problems. I would start with the jetting. Go down on the main jet and see what it does. Also, when is the last time the top-end was replaced? If it’s worn-out, that could be the problem. Has the bike always done this, or did it gradually get worse over time?

  3. great post i liked the part about riding the bike harder because you normally foul plugs at low rpms but i would rather change a spark plug any day rather than blow a piston and possibly have to get the cylinder bored out to..i have a kx 250 1992 and i foul alot of plugs i usually have to change it about every 2 weeks.

  4. I keep flouling plugs every 1 mile, ANY HELP would be great. ZThis is my first bike and ita a 82 YES its old but a 82 honda xr200.

  5. hey i have a 2001 yz 125 and its fouling plughs every time i ride… the top end is like… brand new and same with the reeds… idk if the guy who had it before me had jetted it correctly but it does have an aftermarket fatty and silencer… if its not the jetting then what could it be? help?

  6. i have a 2004 honda cr is it ok to use ordinary 2t oil(not synthetic like those used in scooters)? im ridding the bike on trails. is the ratio mentioned for synthetic or ordinary 2t oil?

    1. Well it depends on the oil. I’d recommend not using just any kind of 2-stroke oil you can find, because it could be for air-cooled engines such as a chainsaw. If it was made for motorcycles, that would be just fine.

  7. ok what does go with a hotter plug? Im running a NGK BR8es in my yz 125. so what plug would you recommend for me to run in my bike?

  8. Ok sorry I met to say BR9es is what I’m using, so if I’m fouling plugs with a br9es should I try br8es? Btw the plug is wet and has carbon on it or at least what looks to be carbon again it’s for my yz 125???

    1. First off, I would get a manual for your bike. If you’ve never done it before, I’d suggest you get some help from a friend that has. You have to take the bowl off the carb and unscrew the jet to see what size it is (should say in small writing on it). If you said the plug was black and wet I’d go down two sizes to start and see what it does.

  9. Ok I found the air screw and I’m gonna mess with that, how many turns Is it suppose to be out? And if I turn it in it will make it rich lee right? And if I turn the screw out it will make it more lean? (in= less air. Out=more air?)

  10. Hey i have a 2001 yamaha 125yz which is a 2 stroke every ride it fowls a plug. If its warmed up it will start after but the next day nope, i am mixing 125 ml of oil to every gallon i believe that is the proper fuel mixture. I am using farm fuel but i use octane booster in it could that be the problem?

    1. That’s a pretty good oil:gas ratio. I wouldn’t use octane booster unless the engine is higher compression than stock. Premium pump gas works just fine (92+) octane. If it’s fouling plugs every ride then you’re problem is most likely caused by poor jetting. Is the plug black and wet after riding? If so, you need to go down a couple sizes on the main jet and possibly the pilot. Do one small change at a time.

  11. starts first kick (when i had one i also lost that first ride) anyhow it fouls plugs when i bought the bike it was on the cheaper side so i had my friend come check it out the bike had a NGK # 7 plug but the stock for my bike was a NGK # 9 the nine fouls very fast as in 1-2 hours riding but the seven will last all day and maybe the next day . when i went to my local bike shop they told me that i was using the wrong plug and i could seize my motor my friend dose alot of work on his bike so i let him look at mine he says it has stock jets what i wanted to know is if i go down in jetting will this harm the bike what are the repercussions? i also have a new air filter (twinAir) is there anything else it could be i could really use some trouble feel free to e-mail me any help would be awesome kylewitteveen1@gmail.com

    1. I wouldn’t recommend running the #7 plug unless you are doing casual trail riding. It will cause the engine to run hotter, and will burn it down if you are riding the bike harder. If it’s fouling plugs with a #9, the jetting is too rich. I’d go down a size or two on the main jet and then see what the plug looks like.

  12. my little malguti was running fine and kick up second time! but it was wet out side and went on grass and the bike was loseing power and splutting and now it nt start!! av empted the tank the carb petrol running in to the carb i pull rev bk the flalp comes up in carb! i kick the bike wth out spar plug and theres power there!! i bured the spar plug and clead it wth wire bruch and it kick up and was reving it but stop reving it and cut out spar plug to hot and black and bit wet! people told me just get a new spar pluk!g! what u thin

  13. hey guys i have a really annoying problem with my 2003 yz 125. i bought it used with a new top end two years ago and ive never had a single problem with it. 7 months ago the spark plugs started to foul. once every couple months the spark plug would foul. so we brought it to the local dirt bike shop(very respectable and smart guys who know what theyre doing) and they rejetted the carb and cleaned everything up. plenty of compression. a month later, BOOM another fouled plug. We brought it back and they again ran a series of tests which all were fine and they tuned and cleaned and checked again. Well weve been bringing the bike back and forth to the shop for probly 8 times now, and all of the men in the shop can not figure out whats wrong. The bike is maticulously maintained. clean filter, clean everything. the guys in the shop ended up taking the whole motor apart and resealing the crank case and honing the cylinder and the whole deal! they put it back together and sure enough after a day of riding, theres another fouled plug.. theyve done everything and the only idea left is to use a special yamaha spark plug for like 30 bucks:/ i just want my baby to live again and run like it did! and by the way i ride the shit out of it so its never low rpms hahahaha thanks -chris

  14. Hello there , i have a kx 80 1986 and my plug is always wet and dark .. Yesterday i wanted to go for a ride and my bike started but only for like 10 seconds then i died , ( i dont use the choke because when its cold and i put the choke on the bike dies ) so i cold start it , i try to rev it a low rpms , and it dies after like 30 seconds , Like if it has no more gas or if it needs to breath. The filtrer is clean ( i washed it in mixted oil ) and sprayed filtrer oil on it . I tried to start the bike without filtrer and it didn’t work ..

    Thanks !

    1. When’s the last time the top-end has been rebuilt? If it still has good compression then the jetting is more than likely causing the plug to foul. You need to lean the jets out and probably the needle as well. If you haven’t done this before, ask a friend that knows how to do it for help. Good luck!

  15. yes it is always black and wet. my dad and i actually adjusted the idle screw and it seems to be running perfect now? weird huh?

  16. it could be because you cant open it up all the way or rolling of (turns off because you step on break with out holding cluch in)

  17. hahahaha no i always start it in neutral. i think its an air problem. when i adjust the idle a little bit, the thing runs perfect all day long but a week later im back to the same problem

  18. hi, i’ve got a early 90s kx250, every 10-20mins of riding my spark plug completely stops sparking, i replace the spark plug straight away and it starts, when they foul because of being to rich is that the szame issue as what ive got? any help will be much appreciated

  19. Ok so i got a 1991 yz 125 and its a very sound bike. i bought three days ago and it ran awesome now it keeps fouling plugs every ride i took the spark plug out and there is black residue on it i am running a 30:1 mixture and when i asked the previous owner what he ran in it he said he ran around a 50:1 mixture what i should i do because i was thinking of taking it to a 40:1 which is more common except for the fact it ran great with the gas the previous owner had in it.

    1. The pre-mix ratio doesn’t have as much to do with plugs fouling as the jetting does. (Read my articles on Pre-mix 101 and Jetting 101). I’d run about 32:1 in a 125cc, and you will want to go down on your main and probably your pilot jet.

  20. Ok so i didn’t get back to your comment until this morning but yesterday i went riding and around my house is alot of sand and i went through two spark plugs just because i laid it over twiice its not the best place to ride having a two stroke because its not enough room to keep the front tire up out of the sand but hey its close to home but nayways my point is why would i go through plugs that quick and when i took out the firs one it had a good layer of oil on it i red your article on jetting tom and plan to go home and fix the jetting but could anything else be wrong? FYI bike runs strong when running it just eats up plugs.

  21. Awesome art. I need some help here, I having the same problem as everyone else – wet fouling… I live quite a bit out of town so to take my bike in is a mission would I be able to set the jets myself or don’t you recommend it if I have never done it before?

  22. Hey; I have a yz 125 2006. I have had the bike for two years now and just this year I have started fouling the plugs. My air filter is good. My top end has less then ten hours on the bike. Would the jetting have changed over the winter because it was not going through spark plugs like this last year. Thanks!

    1. Have you checked the reed valves? Also, pull the pipe off the cylinder and take a peek inside to see if you can see anything without having to take the top-end off.

  23. after 6 months, we finally think we figured out why my yz kept fouling plugs. The stator worked on and off and when it didnt work,there was no spark and the bike would just die right away

  24. hello i have a ktm 125 ,when riding slow for 1 hr max( but try to clear up as much as possible),the plug fouls! i’ve checked carb jetting and air cleaner which are good and the fuel mix is as the manual says. i made a compression test which is 150psi max. i have changed a plug every time i used it can you help ?
    thanks

  25. hi tom, as you already know the main niddle has five groves. when i opened the carb to go leaner i found that the “c” clip was on the top grove ,the leanest. i am conserned on the compression ratio as it is 150psi when it must be 184psi or so i’ve read. also can it be an igition problem? i am tired of buying and replacing plugs every time i go out.
    thanks for you reply

    1. What size pilot does it have? If you’re just putting around, a pilot jet that’s too rich will cause the plug to foul. When the engine is fully warmed up, does it bog or sputter at any certain throttle position? Also, when’s the last time the top-end has been rebuilt?

  26. changed the main (pilot) jet to 165 from 170 to go leaner but still no luck. and yes it bogs at low revs.
    also reeds look fine.

  27. Hi Guys.
    I have a 2006 Yamaha WR250F. Last week it died on me while out on a ride. We found the bike was not getting any spark. After replacing the spark plug, noticed it still did not spark. Turned out the coil pack was faulty. I had the carb cleaned out and set, replaced my air filter, fitted the new coil pack and put a NEW spark plug in. The bike started first time. After idling for 5 min, i turned it off and put the plastics back on. The bike did not start again… Removed the spark plug and tested for spark. The new spark plug was dead. Put another plug in and again it started first time and after turning the bike off, it does not start again… No spark from the plug. Is there anything a can look for? the spark plugs are not black and dont look fouled but they just keep dying :/
    Not sure what to do from here… Please help.

    1. What brand coil pack did you replace it with? Did you try riding the bike at all? This may not have anything to do with your problem, but I would not let the bike idle for more than 1 minute at a time after warming up.

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