Dirt Bike Smoke Explained! To Freak Out or Not?

dirt bike smoke

You just fired up your dirt bike for the first time in a while, and it is billowing smoke.  What does that mean?  Dirt bike smoke is unnerving!  Should you shut it off or just let it warm up?  

It depends on what type of dirt bike you are riding.  If it is a 2 stroke, smoking is normal.  If it is a 4 stroke, smoking at start up MAY be normal.

One thing to look at is the color of the smoke, as that tells quite a bit about what is going on.  It lets you know what is burning, so you can figure out whether or not there is a real problem.

So let’s take a look at why this happens.

Why is my dirt bike smoking?

Short answer? Your dirt bike is burning something other than just gas and air.

When foreign items get into your dirt bike’s engine, it will burn them as part of the combustion cycle and push the remnants out the exhaust.  If you are burning gasoline and air, this exhaust comes out relatively clear.  If you add oil to the equation, as in the case of burning mixed gas for a 2 stroke engine, the addition of the oil will cause the exhaust to be a blueish grey.  White smoke is generally an indication of water.

So your dirt bike is constantly putting out exhaust, but with a 4 stroke, you can’t see it.  Looking at the timing of the smoke and the color of the smoke will give you an idea of where to start diagnosing a problem, if any exists.

Another thing that you will want to try is to figure out where the smoke is coming from.  It could simply be a bad gasket that is causing you to notice the smoke.  For example, on a two stroke engine, if it is smoking out of the cylinder, this is generally a bad exhaust gasket that can be fixed relatively inexpensively.

What causes a dirt bike to smoke on start up?

It is pretty common for a dirt bike to smoke right after you start it up.  The reason for this is condensation in the exhaust.  Think about when you first start a car up, often times there is a little white-ish smoke the stays around for a couple of minutes until everything warms up. 

When you first start a dirt bike, everything will be cold.  As the exhaust starts to warm up, the condensation that is in the exhaust piping will start to warm up as well.  This ultimately causes it to vaporize as steam.  When the entire exhaust system has heated up enough to vaporize all of the condensation, the smoke goes away.

So the white smoke that you see happening right at start up but that goes away after a minute or two is just steam.  This steam is being created outside of the combustion process, so it is harmless.  Don’t worry if this is happening to you, it is completely normal.

This is not restricted to 4 strokes, a 2 stroke will also have condensation in the pipe that gets burned off.  The difference is that you probably can’t tell because of all the oil smoke exhaust coming out of the pipe.

Keep in mind that we are talking about a relatively light amount of white smoke that goes away pretty quickly.  If your bike is billowing white smoke, read on.

My dirt bike has white smoke!

So if your bike is producing a ton of white smoke, you need to turn it off immediately.  This is a tell-tale sign of water getting into the cylinder.  The “water” is actually antifreeze that is leaking around a gasket or a crack in the manifold.  If you catch it soon enough, it might just be a head gasket that is leaking.  While still a pain to fix, it is much cheaper than having to replace a cracked head.

The steam that is produced when you have antifreeze getting burned has a sweet smell to it.  There really shouldn’t be a ton of confusion about whether it is normal smoking (like condensation vaporizing after startup) and a coolant leak because the differences are pretty drastic.  One is light puffs of vapor and the other looks like an old timey steam engine.

Head gaskets don’t generally fail slowly, so it is unlikely that there will just be a trickle of steam coming out.  But if you do have a trickle of steam coming out that continues long after warm up, then you might have a slow leak.

As I mentioned before, shut your dirt bike down immediately and address the problem.  If you don’t you risk causing major damage to the engine.  Check the oil if you have a 4 stroke.  There is a decent chance that it will be milky, which means that there is water in the oil.  Water and oil don’t mix, but when you put them in an engine getting constantly churned around, they do a pretty good job of trying.  This is what creates the milky color of oil.

This can happen to both 4 strokes and 2 strokes if they are water cooled.  Most of your newer motocross bikes are going to be water cooled, so make sure you keep an eye out.

What does it mean when my bike has blue/grey smoke?

Blue/grey smoke is oil burning.  If it is a 2 stroke, this is normal because you have oil in the gas.  If it is a 4 stroke, it means that oil is getting past your piston rings.

Bluish grey smoke on a 4 stroke is not something that you want to ignore, but if it is light then you can probably at least make it back to the truck.  Pistons have rings that allow them to ride up and down the cylinder and create compression.  Piston rings eventually fail if you ride them long enough or are not staying on top of your maintenance.

When the rings go bad, 2 things happen.  First, the oil that should be in the crank case is allowed to slide by the rings.  This will make it where oil is in the cylinder during combustion, which produces the blue grey smoke.  Second, since there is a ring failure, you are not going to have great compression which can cause a multitude of running issues.

4 strokes need to have their pistons changed from time to time anyway, so if you are seeing blue smoke go ahead and put a new piston in there and see if it fixes it.  Chances are it will.

Why is my 2 stroke smoking so badly?

There are a few reasons a 2 stroke can smoke more than normal:

  • The fuel mixture is too rich
  • The oil/fuel mixture is too heavy on oil
  • The power valve (if you have one) is dirty
  • There is oil in the exhaust

There is smoke, then there is SMOKE.  If your 2 stroke is smoking much more than normal, the first thing to check is the gas.  Make sure that oil fuel mixture is right for your bike.  On newer bikes, this is going to be 40 or 50 to 1.  On older bikes it might be as low as 16 to 1.  The amount of oil in the gas has a direct effect on the amount of smoke.  So if you don’t like how much your old bike that calls for a 16:1 oil/fuel ratio is, then there isn’t much you can do besides buy a new bike.  Never run a 2 stroke with unmixed fuel or you will kill the engine quickly.

The second cause of excess smoke is the power valve.  Some 2 strokes have a power valve that is designed to help smooth out the power curve of the bike.  These get dirty over time and need to be cleaned.  If you have the right oil mixture and are still getting excessive smoke than go ahead and clean the power valve.

The last thing that might be going on is your exhaust piping is just gummed up with oil residue because it hasn’t been cleaned in a while.  If the power valve doesn’t fix it then cleaning the exhaust might.

Finally, the mixture on your carb might be off.  If you are running rich or have your choke on, there is going to be more oil present every time your spark plug fires, so there will be more smoke.

Wrap Up

When I first started riding dirt bikes, they always made me a little nervous because it always looked like the engine wasn’t running correctly.  Once I learned what the colors of the smoke meant and what caused them, I was able to calm down some. 

2 Strokes are going to be smoky, there is no way to avoid that.  4 strokes can smoke a little at start up when the condensation is burning off, but after that should be clean.  If you take nothing else away from this article, remember that white smoke is water burning off, and blue smoke it oil burning off.  Start from there and figure out what is really going on with your bike.  Ride hard and stay safe!

2 Stroke vs 4 Stroke – The first part of this article looks at how the two types of engines work.
Animation of a 2 Stroke Engine
Animation of a 4 Stroke Engine