Why Does My Outboard Motor Backfire?

A backfiring outboard motor is a symptom of a wider problem with the motor and therefore means that you should look to fix this as soon as you can.

The loud, periodic bangs from the motor when backfiring are hard on the ears but, if left untreated, mean that your motor could be damaged from the source problem that results in backfires.

A rich fuel mixture or a vacuum leak causing a type of splutter called a ‘lean sneeze’ are two of the main reasons for an outboard to backfire. Note that spark plug issues, although possible, are likely not the cause of backfiring.

Let’s first have a look at the likely main culprit, that of your outboard motor running ‘lean’ on fuel, leading to a backfiring engine.

Why do Outboard Engines run Lean?

A vacuum leak in the engine means that more air gets into the engine and affects the air to fuel mixture, so it runs lean on fuel but has excess air. Faulty ignition coils are one possible reason why your outboard motor is running lean. These generally fail fairly regularly so this could be one cause. 

What does Running Lean Mean in an Outboard Motor?

An outboard running lean is when the air to fuel ratio in the cylinder has too much air and not enough fuel, meaning that it struggles to push the piston properly and therefore just performs poorly.

Since all combustion engines work on just the right air-fuel ratio, this causes a problem and will mean reduced performance until you fix it.

Vacuum Leak and Lean Sneeze on an Outboard Motor

A ‘lean sneeze’ is when an outboard’s RPM goes up and down quite quickly, and also splutters or sneezes.

To fix this, take a gas torch from an LPG gas cylinder and place it at the point at which you think the vacuum leak is happening, around the crank case, for instance, and move it around until you hear a change in RPM in the running motor.

This works because it is replacing the extra air getting in with gas, meaning that the air-fuel ratio is improved temporarily and thus giving an indication of whether you have found the source of your vacuum leak and lean problem.

As per usual, Dan at Dangar Marine has an excellent YouTube video on this topic, as shown below.

Vacuum leaks could happen in many places, such as the bottom crankcase seal, upper crankcase seal or the engine block, so you need to be open to the possibility of it being in one of many sources. 

What does it mean when a 2 stroke engine backfires?

2 stroke engine backfiring signals a problem upstream with your motor, which could be due to a fuel mixture problem, possibly running rich, a vacuum hose being loose, or faulty spark plugs.

Unless you know what you are doing mechanically, it is best to take it to a local mechanic who can help you diagnose the problem and get it solved because it is not easy to identify and will take a long time.

Can a bad spark plug cause a boat’s engine backfire?

Yes, bad spark plugs can cause an engine to backfire, although it is not usually the main source. It is therefore a good idea to look at the other causes of an engine backfire before changing the spark plugs in your engine.

This is commonly recommended as a first place to start since the spark plugs are fairly easy to change and many people know how to fix them, so they like to do what they can.

Furthermore, it is recommended to change the spark plugs on your outboard every season, so if you haven’t done that in a while then you may as well do so as part of the regular maintenance anyway.

Spark plugs by NGK over on Amazon are recommended and are inexpensive, at around $5 each.

Check the Fuel Mixture in Your Outboard

Since the engine relies on the right mixture of air and fuel getting into the cylinder, you should try to identify whether this is causing the backfiring from your motor.

Signs that your engine is running rich include strange smells, increased fuel consumption, idling issues, spluttering and of course backfiring.

If you have any of the other warning signs of your outboard running too rich, then it is worth looking to fix those problems as any one of them could fix the backfire issue.