Sea-Doo Rotary Valve Explained [Video]

The rotary valve is the fuel intake valve on vintage 2-stroke Sea-Doo engines. This spinning metal plate is indirectly driven by the crankshaft through the rotary valve shaft. The main idea behind Sea-Doo’s rotary valve is to control the intake’s opening and closing. Therefore, it does the same job as the intake valve system on 4-stroke engines.

If you want to find out more about this advanced Sea-Doo valve design, this post is for you.

We at JetDrift have compiled the basics of this unit all under one roof.

If you own a 2-stroke Sea-Doo that has rotary valve malfunctions, or you just want to figure out how to time the valve plate properly, you may find this post useful.

What is the Rotary Valve on a Sea-Doo?

To put it simply, the rotary valve on Sea-Doos refers to the fuel intake valve of their 2-stroke Rotax engines. This valve system is timed to the crankshaft and intended to control the intake of the fuel-air mixture.

Certain 2-stroke Sea-Doos, the ones with 947 and 951 engines, feature a simpler reed intake valve.

This system controls the intake charge better since it allows the fuel-air mixture to flow into the cylinders without any restrictions.

You can find the rotary valve on the side of the engine, right under the rotary valve cover.

How Does a Sea-Doo Rotary Valve Work?

The rotary valve is a large half-moon-shaped plate that is hidden under the rotary valve cover. The valve plate is driven by the rotary valve shaft, which is connected to the center of the crankshaft via a worm gear.

Therefore, the rotary valve shaft and the crankshaft are spinning in correlation.

If timed properly, the rotary valve makes one of the two intake ports accessible to let the air-fuel mixture into the engine. Then the valve closes the intake port to keep the mixture in the cylinder.

The main idea behind this process is to produce a better mixture flow and prevent blow back issues.

The key advantage of rotary valves is that they let the intake port be completely open without any restriction.

This more efficient, unrestricted mixture flow converts to improved power density as well as a more rapid combustion.

Eventually, this provides increased engine power within a wider RPM range.

Which Sea-Doos Have Rotary Valve?

Interestingly enough, Bombardier originally developed its rotary valve Rotax engines for Ski-Doo snowmobiles.

This is because reed valves didn’t work well in cold weather, so the manufacturer upgraded them to the more advanced rotary valve system.  (Contrary to popular belief, rotary and reed valve systems are not interchangeable.)

The new technology was not only more efficient in frozen environments but it also proved to be more effective, so it quickly found its way into Sea-Doo PWCs as well.

As a rule of thumb, Bombardier used three different rotary valves in their Sea-Doo engines, the 132-, 147-, and 159-degree valves.

The 132-degree rotary valve plate was used in early Sea-Doo rotary engines, and used in the following models:

·         1989-1991 SP

·         1990-1991 GT

·         1991 XP

·         1997 SP

·         1997 Sportster (jet boat)

In the 1990s, the 147-degree rotary valve plate became more prevalent in Sea-Doo engines. This valve plate configuration was used in the following Sea-Doos:

·         1991-1996 SP

·         1992-1996 GTS

·         1992-1994 GTX

·         1996 GTI

·         1992-1994 XP

·         1993-1996 SPI

·         1993-1994 SPX

·         1993-1997 Explorer (jet boat)

In contrast, the 159-degree valve plate was used in the more advanced 2-stroke Sea-Doo engines including 657, 717, 720, 787, and the flagship 800.

Based on our research, the 159 valve plate fits on the models below:

·         1995-1997 GTX

·         1997-2001 GS

·         1999-2000 GSX RFI

·         1997-2004 GTX RFI

·         2005 GTI 3D RFI

·         1997 GSI

·         1997 GSX

·         1996-2005 GTI

·         2002-2004 GTI LE

·         2003-2005 GTI LE RFI

·         1996-2001 GTS

·         1995-1997 HX

·         1995-1999 SPX

·         1994-1997 XP

·         1995- XP 800

Besides PWCs, Sea-Doo used rotary valve engines in many 2-stroke jet boats like the Speedster, Sportster, Challenger, and Explorer.

(These lists are for informational purposes only. For more information, please refer to your ski’s service manual.)

Conclusion

The rotary valve is the fuel intake valve on vintage 2-stroke Sea-Doo engines manufactured by Rotax.

The heart of this system is a half-moon-shaped metal valve plate, which is driven by the rotary valve shaft. The latter is connected to the center of the crankshaft between the cylinders via a worm gear.

The motion of this plate opens and closes the intake port at the right time.

Unlike reed valves, the rotary valve allows the fuel-air mixture to flow into the cylinder without any restriction.

This efficient design provides an increased and precise airflow control, which converts into more engine power and higher RPMs.