Hi, Joey assuming it's a 2 stroke the usual suspects are:
1. The 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 a fine mist of some air filter oil or spray. "WARNING" adding too much oil can also clog up the filter, resulting in another fouled plug.
2. 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, the 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 in good condition.
3. 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. Keep an eye on the plug color you're looking for a light tan color anything whiter and you could burn a hole in your piston. Spark plug manufacturers aren't always the same, so make sure you find out how they rate them.
4. 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 on jets.
5. Piston rings badly worn or broken.
6. Faulty ignition coil low output.
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