10 Reasons Why Your Jet Ski is Backfiring [Fixing Guide]

Internal combustion engines are prone to backfiring, and jet ski engines are no exception. The most common reasons why a jet ski would backfire are as follows:

1. Bad engine timing
2. Sheared woodruff key
3. Rotary valve out of timing (Sea-Doos)
4. Spark plug and wiring issues
5. Carb and fuel line malfunctions
6. Bad fuel
7. Air intake problems
8. Damaged reed valve
9. Real crank seal leak
10. Other types of engine or exhaust system damage

If you want to find out more about these malfunctions and their fixes, you’ve come to the right place.

We at JetDrift have compiled all you need to know about jet ski backfiring into this post!

What is Backfiring on a Jet Ski?

Backfiring on a jet ski occurs when the fuel ignites outside the engine cylinders, either on the intake or exhaust side. As the name suggests, backfiring can cause flames in the throttle body or the exhaust pipe, which is rarely seen on a jet ski due to its unique exhaust system. In most cases backfiring on a jet ski can only be recognized by the popping sound or a single loud boom, accompanied by some loss of power.

Backfiring was much more common on vintage 2-stroke jet skis, but rarely happens on modern 4-stroke models.

Since these machines come with a fuel-injection system, the fuel-air mixture is controlled by a computer instead of carburetors.

Backfiring can be dangerous to your jet ski as it can damage the engine and other components. Also, backfiring typically robs engine power and increases fuel consumption.

Therefore, a jet ski that repeatedly backfires has to immediately be inspected and serviced.

Let’s drill into the details and see what causes a backfire, and how you can fix these issues!

10 Causes of a Jet Ski Backfire:

1. Bad Engine Timing

There’s no question that the most common reason for a jet ski backfiring is bad engine timing.

Inside the engine, the fuel-air mixture has to be ignited at the exact right time. These precise explosions drive the crankshaft, providing power to the jet ski.

Bad engine timing is when fuel ignites in the cylinder at the wrong time. If the timing is wrong, this also means that the valves are not in the correct position when ignition occurs.

When there is delayed timing, unburnt fuel can go to the exhaust system through the exhaust ports. When this extremely hot fuel mixes with fresh external air it will explode in the exhaust.

Also, the jet ski can backfire through the carburetors as well.

2. Sheared Woodruff Key

Bad engine timing (and thus backfiring) on a jet ski is often caused by a sheared woodruff key (a.k.a. flywheel key). If this key is sheared, the flywheel can rotate freely and not with the crankshaft.

This malfunction leads to a desynchronized engine as the valves won’t open at the right time.

3. Sea-Doo Rotary Valve Out of Timing

The rotary valve is the fuel intake valve on 2-stroke Sea-Doo engines, powered by the crankshaft through a set of gears. The rotation of this valve opens and closes the intake port in time. If the rotary valve is out of timing, it often causes poor performance and backfiring.

If your 2-stroke Sea-Doo backfires, you may want to look at this rotary valve, especially if you’ve recently taken the engine apart.

Check your ski’s service manual to ensure that the valve was correctly installed.

It’s also recommended that you remove the valve shaft and inspect the brass gear that drives the rotary valve.

This brass gear matches a steel gear nestled inside the engine.

The splines of the brass gear are prone to shearing off, causing the valve to be out of time.

4. Spark Plug and Wiring Issues

If the engine and rotary valve’s timing look good, it’s time to check the spark plugs and ignition wiring.

First, make sure that the plug wires are not switched.

If the wires are in the right order, inspect the spark plugs, the ignition system’s entire wiring, and the plug caps for damage.

5. Carburetor and Fuel Line Malfunctions

Incorrect carburetor settings and fuel line malfunctions can also cause a jet ski to backfire.

This is because this malfunction is often caused by a too rich or too lean a fuel-air mixture.

A lean mixture (too much air in the mixture) can produce backfires in the intake manifold, so the flames can go through the carburetor.

In contrast, a rich mixture contains too much fuel, which does not completely burn in the cylinder. The remaining fuel vapor goes into the exhaust system along with exhaust fumes, causing a backfire.

An improper air-fuel mixture can be caused by various malfunctions, including improper carb settings, clogged fuel filters, a vacuum leak, or a faulty fuel pump.

Only a creased diaphragm in the carb or fuel pump can lead to an insufficient fuel supply!

This is why you should check the entire fuel line, including:

• Carburetors and injectors
• Fuel lines and valve
• Fuel filter
• Fuel pump
• Tank

If you have a fuel-injected jet ski, keep in mind that the injectors must be regularly checked and maintained.

Unfortunately, the fuel system of a fuel-injected jet ski engine is much harder to troubleshoot.

6. Bad Fuel

If the fuel system looks good, it’s worth checking the gas itself. Only a small amount of water in the gas can foul the engine.

Drain your tank and refill it from a trusted source to make sure the fuel is completely clean. (Don’t forget to clean the carbs, as water can accumulate in them.)

Note that different engines require different octane ratings, especially high-performance ones, which are very picky about gas. You can check the recommended fuel specifications in your ski’s manual.

If you have a 2-stroke ski, you should also pay attention to the right oil-fuel mixture, as running too rich can also cause the engine to backfire.

7. Air Intake Issues

To avoid backfiring issues, make sure that the flame arrestor and the air filter and (if present) are clean and in good shape.

The flame arrestor is a metal mesh designed to stop flames from the engine caused by a backfire.

This is an essential safety feature as gas vapors are prone to accumulating in the hull of jet skis. If these vapors meet the flames coming from the engine, it could even end in a serious explosion.

This happens very rarely, but there’s always a chance!

Flame arrestors should be inspected regularly. The fastening strap may loosen, and the metal mesh can get damaged, allowing more air into the engine.

In contrast, a clogged air filter or flame arrestor blocks incoming air, causing the engine to run rich.

Both issues can result in backfiring!

8. Rear Seal Leaking

Another common reason for a jet ski backfiring is air leakage due to a broken rear crank seal. The rear seal around the crankshaft is prone to popping out over time, allowing extra air into the engine.

This air leakage can lead to backfiring or a seized engine in the worst-case scenario.

First, you should visually inspect this seal, but you should also pressure check the engine for an accurate result.

9. Broken Reed Valves

Damaged reed valves can also cause malfunctions like poor engine performance or backfiring.

Reed valves can be stuck, bent, or even cracked, which leads to a lean fuel-air mixture. As we’ve discussed, this can cause backfiring on the intake side.

You need to remove the reed valves, clean and carefully check them to inspect for this issue.

10. Other Engine or Exhaust System Damage

Other than the above, backfiring can be caused by other engine or exhaust system malfunctions.

Inspect the engine block, gaskets, seals, and exhaust system, including the pipe, exhaust manifold, and waterbox.

You also can’t go wrong with a compression check on the engine.

If you can’t find the cause of the error, it’s time to take your ski to a professional. Keep in mind that backfiring can cause engine issues in many ways.

On top of that, it could be a symptom of more serious damage, which likely needs immediate repairs.

Takeaways

Backfiring on a jet ski is when the gas-air mixture explodes outside the combustion chamber.

Vintage 2-stroke jet skis are more prone to backfiring, as their engines are not computer-controlled.

This malfunction always comes with recognizable loud popping sounds.

A jet ski engine can backfire through the intake manifold or the exhaust, depending on the source of the problem.

In a nutshell, the most common reasons for a jet ski backfiring are as follows:

1. Bad engine timing (most common reason)
2. Sheared woodruff key
3. Rotary valve out of timing (on Sea-Doos)
4. Spark plug and wiring issues
5. Carb and fuel line malfunctions
6. Bad fuel
7. Air intake problems
8. Damaged reed valve
9. Real seal leaking
10. Other engine or exhaust system damage

This is our short guide about the backfiring of a jet ski engine, we hope you find it useful!

Disclaimer: This post is for general information purposes only. Always read your ski’s service manual carefolly before you do any maintenance.