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Why Does My Outboard Motor Backfire?

Why Does My Outboard Motor Backfire?

So you’re out on the water, enjoying a nice, relaxing day of fishing when BAM! Your outboard motor backfires.

Not only will a backfiring engine scare all the fish away, but it could be a sign of various underlying problems. 

So, you may be wondering: what causes an outboard motor to backfire? And what should you do about it!

Keep reading! In this article, we’ll discuss the various factors and issues that could be causing your engine to backfire.

5 Reasons Why Your Outboard Motor is Backfiring

First off, we should point out that it’s not always possible to self-diagnose your engine problems. If your outboard motor is backfiring, the best thing you can do is seek professional help.

Take the motor to your local boat engine specialist; they will be able to run numerous high-tech tests and diagnostics. This way, they will be able to properly diagnose what’s going on. If you try to do it yourself, chances are, you’ll just be guessing.

That said, if you want to try and figure it out on your own before taking it to a specialist, there are some things you can look for. The most common reasons why an outboard motor might backfire include:

1. Too Much Fuel

your motor is being flooded with too much fuel

If your motor is being flooded with too much fuel, it is probably going to backfire. But why is it getting too much fuel? This root problem is most likely related to the carburetor and is caused by bad ignition timing. 

A motor needs an exact amount of fuel and air mixed for combustion to work as it should; if too much fuel gets in and is not able to mix with the same amount of air, the fuel floods the chamber because it can’t all be burnt off quickly enough.

This excess fuel may go shooting out of the exhaust port, or it may combust too late while flowing through the exhaust chamber. This belated combustion produces the loud noise associated with backfiring.

It’s hard to self-diagnose this particular problem unless you are skilled at working on small engines and know your way around the carburetor.

The better option is to get it checked out professionally. If your outboard motor backfires repeatedly and you notice flames shooting out of the exhaust port, you may be experiencing this problem, and now would be a good time to get it checked out.

2. Too Little Fuel

Too little fuel can also cause your outboard motor to backfire.

Over time, fuel pipes may get clogged with old fuel or debris such as dirt and rust. This will constrict the flow of fuel more and more as the debris continues to build up. 

outboard motor with too little fuel

Eventually, the buildup may prevent appropriate amounts of fuel from reaching the engine. 

If not enough fuel is getting through, the engine may sputter or backfire. The pipes will need to be cleaned and unclogged to stop repeated misfirings.

This may be your problem if your outboard motor is older or hasn’t been serviced in a long time, or if it hasn’t been properly cared for over its lifespan.

You may be able to check your own fuel pipes; if they are dirty or clogged, you will need to take your outboard motor to an engine specialist and allow them to clean and service them for you.

How to check fuel flow on outboard

3. Dirty Fuel Tank

A dirty fuel tank can not only cause your motor to backfire, but it may lead to clogged fuel pipes as well. 

Rust and other debris may build up in the fuel tank over time. This is especially true if you repeatedly let your fuel levels drop extremely low before refilling. 

When fuel levels drop, the small amount of fuel left in the tank may mix with rust residue on the bottom. 

This can cause the fuel itself to become dirty, which may in turn cause issues with the fuel pipes as well as wear and tear or buildup in other parts of the motor.

These are all reasons why a dirty fuel tank can cause your engine to backfire. You may be able to clean it yourself, but the better option is to take it to a professional small engine mechanic.

Sportfisher 5 Hp Outboard Pt 14 Clean Rusty Fuel Tank

4. Spark Plug Issues

Spark Plug Issues

Repeated backfires from your outboard motor may be caused by bad spark plugs or wires.

Backfiring may simply be caused by a wire or plug that has gone bad or sustained damage; in this case, you’ll need to replace the faulty plug or wire.

Your motor may also backfire due to improperly replaced wires after you’ve changed out the spark plugs–if you got the wires crossed or didn’t put the right ones back with the right plugs. 

Faulty or improperly connected spark plugs cause your motor to backfire because they are not initiating the proper steps at the proper times in the ignition process.

If you suspect this is the problem, check your spark plugs; make sure they are all wired correctly. 

How to Test Spark Plugs on an Outboard | Outboard Ignition System PT 2 |

Double-check that all spark plugs and wires are free of damage and corrosion. If you’re not sure what to do or how to do it, take the motor to a specialist.

5. Shear Pin Damage

On an outboard motor, the shear pin attaches to the propeller and may break if the propeller is stressed or damaged. 

This is a safety feature built in to protect the motor from sustaining more extensive damage.

A broken shear pin will cause your engine to backfire the moment it happens; so, if your motor seems to have backfired in response to your motor colliding with something underwater, you can assume the problem was caused by the shear pin breaking. 

You might be able to diagnose and fix this problem on your own if you’re familiar with the parts of your engine. Look for a broken propeller or a shear pin that has snapped.

How to fix a broken shear pin for the leadallway 4hp outboard

Again, if you’re unfamiliar with your engine’s anatomy or not sure what to look for, call a professional to diagnose what is causing your motor to backfire.