Why Does My Boat Motor Smoke

I just launched my boat for a day of fishing at a local lake.  The late spring morning is calm and cool as I crank start my small, two-stroke motor.  As soon as it purrs to life, a thick cloud of smoke spews out of the exhaust and surrounds the boat.  At that point I started to wonder, why does my boat motor smoke and what can I do to fix it?

Taking an outboard motor to a specialist can be expensive, so I did some research and this is what I learned.

Boat motors, especially two-strokes, commonly smoke because of an improper fuel to oil ratio or when the motor runs rich on fuel.  When this happens, excessive blue-tinged smoke will come out the exhaust port, indicating too much oil is burning directly in the combustion chamber.  If instead, the motor has only small amounts of white smoke, this may be normal and a little smoke from condensing exhaust gases, as it meets cool damp air, is expected.  When the motor gets to operating temperature the smoke levels should decrease. 

Now let’s take a closer look at what’s happening and what you can do about it.

What color is the smoke

Not all smoke is created equal.  The color, in addition to the quantity, of the smoke emitted by your boat motor can tell you a lot about what is going on.  Here is a quick synopsis to help diagnose your issue.

White smoke or vapor-  Generally, small amounts of white smoke do not immediately indicate an issue.  Most of it may be normal water vapor condensation.  If the motor runs fine at idle, as well as full open, then the smoke should not be too worrisome.  Once the motor gets warmed up, less smoke will be made.  Water vapor will also decrease as the day gets warmer.

Copious amounts of white smoke and steam in tandem with a rough running motor could indicate more major issues.  Cracked cylinders or bad valves that let water from the cooling system enter the combustion chamber could wreak havoc on a small engine and should be taken to a professional for repair or advice.

Blue smoke-  Occurs anytime oil starts to combust in high volume.  It appears as slightly blue smoke from the exhaust.  It is not necessarily bad because some oil is expected to be consumed this way in a two stroke.  When your oil to fuel ratio is incorrect, more smoke than normal may be seen.  Throw out the bad fuel mix and start over.  You can also use synthetic oils that smoke less.  We will discuss this later in more detail.

However, if you continually have billowing, dense blue, clouds of smoke when running the motor, more serious oil related problems are to blame.  An outboard motor mechanic should be your next call.

Black smoke-  “Houston we have a problem.”  A cloud of black smoke is obviously not normal and definitely suggests severe damage has occurred.  Immediately shut off the motor.  Have it inspected promptly before starting or using it again.

What is the right oil and fuel mixture

Two stroke motors stay lubricated by forcing oil directly into the combustion chamber.  Every combustion cycle has a bit of oil injected and this keeps everything running smooth.  This is why most two stroke outboards require fuel pre-mixed with oil.

By the very nature of this lubricating method, some oil will end up partially combusting and appears as smoke out the exhaust.  However, you start to run into problems when things get out of balance. 

When determining the proper oil and fuel mix, always follow the recommendation of the motor manufacturer.  A common mix ratio is 50:1.  This is 50 parts fuel to 1 part oil.  It is difficult to get the mix exactly perfect.  The two stroke oil jug will have quick guides that suggest the volume of fuel and oil needed to make the proper mixture, but it is easy to get this wrong.

If you add too much oil by mistake, the increased amount of oil in the combustion chamber will lead to more blue smoke and a buildup of carbon deposits.  Overtime, if left unchecked, these carbon deposits lead to poor performance of the motor.  Not to mention more smoke.  

Sometimes it is not your mix ratio that is causing a problem.  Not all oils are created equal.  Using quality oil will go a long way to drastically reduce the amount of smoke your motor pumps out.  

Try using synthetic oil.  Evinrudes ETEC XD-100 is well known to nearly eliminate smoke due to oil combustion.  While it is expensive, it is a small price to pay for a more pleasant day on the water.  Other brands of synthetic oils are available and you may want to experiment to find what works best for your particular motor.

Also, check that the fuel and oil mix you are using is not old.  Avoid using fuel from last season and start with a fresh mix.  

How much smoke is normal

Lots of smoke is never good, but some smoke is expected and it may actually be mostly water vapor condensing as it exits the exhaust.  Water is a natural byproduct of combustion.  If you have ever started your motor on a cold morning, then you probably noticed that there is more smoke or vapor.  During the warmer days of summer, you should see less vapor condensation.  

A quick test can help decide if you have an excessive smoke problem or if it is simply water vapor.  While the motor is running, watch the exhaust cloud.  If it dissipates quickly, it is condensation.  If it lingers for more than 30 seconds, it is most likely smoke.

Other common issues

Fouled spark plugs are often associated with poorly running motors and increased smoke.  When carbon deposits build up from oil and fuel residue, it eventually begins to cake on the spark plug.  A fouled plug does not make a nice hot spark, which leads to incomplete burning of fuel. 

Part of regular maintenance should include inspecting the plugs.  With a spark plug wrench, remove the plug and visually check for a thick, sooty layer or chunks of carbon material anywhere on the electrode.  

Use a small wire brush to clean the plug and follow up with a gap check.  Keeping a proper gap on a spark plug will ensure you get the hottest spark possible.  A gap gauge can be found at most automotive stores.  Follow your motor manufacturers specifications when setting the gap.  If your plug is beyond repair, just replace it with a new one.  

Some outboards are just more finicky and will reward you with rough starts and more smoke when stored or shut down improperly.  

When storing your two stroke motor away for the season, the goal is to prevent damage caused by old fuel that contains ethanol.  Discard all the old fuel from the motor and run it until it is dry. 

Then, follow the manufacturers instructions, if necessary, to remove the carburetor and drain the float valve of all fuel.  Your motor will last longer and run cleaner when you start it up for the first time next season.

Finally, a motor that runs too rich on fuel will smoke more because the increased fuel to air ratio prevents a complete burn during combustion.  The unburned fuel and oil is coughed out as a hot cloud of smoke.

Many outboards have two idle adjustment screws that are used in combination to achieve the proper mix of air and fuel.  You should follow the instructions of your particular motor when making any adjustments yourself.

What about four stroke outboard engines

Like your truck or car engine, a four stroke does not combine oil directly into the fuel. Instead, they utilize an oil sump that “splash” lubricates the engine parts as the cam shaft rapidly moves through the engine cycles.  Four strokes, therefore, do not suffer from the same smoking exhaust that plagues the direct oil burn of two strokes.

Similar to starting your car before work on a cold morning, a four stroke outboard will have minimal white smoke but a moderate amount of water vapor.  As it gets to operating temperature, nearly all the visible exhaust will disappear.

If your four stroke smokes more than you think is normal, definitely get some expert advice.

Final thoughts

It may seem daunting to deal with small motor problems, especially when you would rather be fishing, but with a bit of knowledge and the tips in this article, you can be back on the water with less smoke and a happy motor.  Like all your other fishing gear, proper boat motor maintenance and a little TLC will go a long ways in keeping you fishing longer.