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5 Reasons Why a Jet Ski Revs Out of Control [Video]

5 Reasons Why a Jet Ski Revs Out of Control [Video]

There can be many reasons why a jet ski revs out of control, but the most common ones are as follows:

  1. Air leak
  2. Carb malfunctions
  3. Exhaust valve issues
  4. Stuck throttle cable
  5. ECU malfunctions
If you want to find out more about these malfunctions and their remedy, this post is for you.

We at JetDrift have compiled all you need to know under one roof!

Why Does Your Jet Ski Rev Out of Control?

Let’s face it, it can be pretty scary when a jet ski revs out of control! It’s a common issue with 2-stroke jet skis, especially those equipped with carburetors.

When it happens, the engine automatically revs up to the max RPM without you even touching the throttle.

In the worst-case scenario, the engine reaches or even goes over the rev limiter and you can’t stop it unless by cutting the fuel.

As you can imagine, this can be very stressful and dangerous especially if it happens while riding. Since the ski seems unstoppable, the issue is often referred to as the “runaway problem.”What’s more, during this malfunction the throttle is closed, which doesn’t allow the oil pump to work. If the malfunction persists for a long time, the engine will drain all the oil out of the lines, resulting in seized pistons.

To avoid these risks, runaway issues have to be immediately investigated and fixed.

Let’s see how it’s done!

5 Common Reasons Why a Jet Ski Revs Out of Control

1. Air Leak

It’s safe to say that the most common reason why a jet ski revs out of control is a small air leak. Air can leak into the fuel system and engine at several points, including:

  • Between the reed plate and carb
  • In the carburetors
  • Carburetor gasket
  • Fuel lines
  • Fuel valve
  • Rotary valve
  • Cylinder base gasket
  • Crankshaft seals
  • Case halves
  • Reed sealing
Consider inspecting all of these components to find the source of the leak. Sometimes a vacuum leak generates an audible noise like a whistle or hissing sound.

Otherwise, you may want to spray some brake or carburetor cleaner on the suspected areas with the engine running. Start around the carb and the reed plate.

If the engine revs down or even returns at idle speed, you’ve found the leak.

The key is to only run the engine for very short periods since revving up to the red line can damage the engine in many ways. What’s more, since jet skis feature a direct drive system, a high-revving engine spins the impeller pretty fast.

Running the ski for too long out of the water at high RPMs may result in serious jet pump damage.

Also, don’t forget to hook up the cooling system on the hose to run fresh water through the engine.

If you can’t find the air leak with the “spray trick,” your other option is to do a pressure check on the engine.

2. Carburetor Malfunctions

Carburetor malfunctions can also cause the jet ski to rev out of control.

Besides the aforementioned air leak issues, carburetors can go wrong in many ways. For example, the butterfly can get stuck open, which provides the same result as if you kept the throttle open.

Improper carb settings can also cause engine malfunctions and over-revving issues. You may want to adjust the carb to find its sweet spot. Note that certain engines like the Rotax 951 are prone to revving out of control if the idle speed is set too high.

If this doesn’t help, it’s time to pull out and carefully clean the carbs. Sometimes the problem is caused by a clogged jet or screen in the carbs.

Consider replacing the gaskets, or even rebuilding the entire carbs if needed.Note that the smallest wrinkles on carb gaskets can cause air leaks! This is why a new gasket kit can solve air leakage issues in many cases.

For the most effective sealing, you may want to apply some liquid gasket sealer when assembling the carburetor.

You can even do a leak test on the carbs as well to make sure they’re working properly.

Fuel-injected 2-stroke jet skis are much less common than their carbureted counterparts. On these skis, the injectors and the fuel system can also suffer from a vacuum leak.

Due to their complexity, troubleshooting an EFI jet ski engine requires the right skills and some special tools as well.

3. Exhaust Valve Issues

It’s a lesser-known fact that a defective variable exhaust valve system can also cause the jet ski engine to rev out of control.

These innovative power valve systems like Sea-Doo’s RAVE and Yamaha YPVS appeared on the high-performance 2-stroke jet skis in the mid-’90s.

The very first jet skis with variable exhaust valves were the 1995 Sea-Doo XP 800 (Rotax 717 RAVE) and the 1998 Sea-Doo GSX (Rotax 947 RAVE).

Due to their complexity, these valves require periodic maintenance and cleaning. You can read more about variable exhaust valves and their maintenance in this post.

4. Stuck Throttle Cable

It may seem obvious, but a stuck throttle cable can also shoot the revolution of the engine through the roof.

Vintage 2-stroke jet skis feature a regular mechanical throttle cable, which can become sticky for many reasons, including:

  • Cable settings are too tight
  • Lack of lubrication
  • Worn-out throttle cable
  • The cable is bent in the hull

5. ECU Malfunctions

Modern jet skis are manufactured with a throttle-by-wire system, meaning that they feature an electric connection between the engine and the throttle.

The signals generated by the throttle lever go through the main computer, known as the ECU. And just like any other part of the jet ski, the ECU and throttle sensor can also break down, causing improper engine operation.

ECU and sensor failure are often accompanied by fault codes that can be read and identified by a scanner tool.

Conclusion – Why Are a Jet Ski Revs Out of Control?

It’s safe to say that the main reasons why a jet ski engine revs are out of control are an air leak or a defective carburetor. However, a sticky throttle cable or exhaust valve malfunctions can also cause similar symptoms, just like ECU and sensor issues.

If your jet ski is prone to “running away” on its own, it’s highly recommended that you fix this issue immediately.

A high-revving jet ski engine can easily produce a seized piston or other severe damage!