Two-stroke motors are quite simplistic compared to their four-stroke cousins. They are lightweight, make large amounts of power and they don't have the complex intake and exhaust pieces of a four-stroke.  While those complex pieces aid to the efficiency of the motor, modern snowmobile motors use a reed valve to help manage the intake charge.

Reeds are a one-way valve, located downstream from your carbs and mounted between the crankcase and the cylinders. This allows the motor to pull the air/fuel mixture from the carbs, through the reeds and down into the case. The reeds stop the air/fuel mixture from backing out of the case and up into the carbs again while the internals of the motors are spinning around. With a reed valve motor more (as opposed to a piston port motor) the intake charge will stay inside the motor more reliably.

A stock motor showing the airbox, carbs, reed valves and cylinders

The reeds are mounted somewhere between the cylinder and the crank case.

As these are integral to the design of the motor you will want to make certain that your reeds are in top working condition. If they not you will have a poor running sled at slow speeds and your motor will not generate the top end power that it once did. To inspect or replace reeds you first to remove them from the cases and this is done by removing the air box, carbs (or throttle bodies on an EFI sled) and then the reeds themselves.

The reeds have now been removed from the case and are ready for inspection

As mentioned above the reeds are a one-way valve that allows the motor to breathe through them when it is drawing in the air/fuel mixture. When the motor moves to compression stroke the pressure that is created in the case will force the reeds back against the reed blocks forcing them to seal and thus holding the intake charge in the case so it can be pushed into the cylinders.

Inspecting the reeds is quite easy. Make sure there are no chips in them and that the reeds seat (or almost seat) against the reed blocks.

While my reeds looked fine I decided to upgrade my stock reeds to VForce reeds. I choose VForce mainly on design. As you can see from the picture below, the VForce reeds have twice as many inlets as the stock Polaris reeds which means that each reed petal has to move half as much as opposed to the Polaris reeds. This will allow more air to travel through the reed blocks when you are wide open throttle (thus making more power). In addition, the reed petals will be able to seal faster when the motor moves to compression stroke which means more efficient low speed operation.

Here you can see the difference between the stock reeds and the VForce reeds. Notice the VForce reed cages have twice as many reed petals, in a W formation, as opposed to the stock Polaris reeds.

Another shot highlighting the difference between the two. Note that the VForce reeds have no little screws on them to shake loose and go flying though the motor and damage it. They are also made from a composite material that looks great!

Installation is simply reverse of removal. Place the reeds back in the case (with any appropriate gaskets), reed stuffers, the carb boots; carbs. Don't forget to connect the throttle, choke and any oil lines and airbox. Be sure that the airbox seats to the carb bells properly as you don't want any hot, moist air leaking between the carbs and the airbox and going directly into the carbs.

Once I installed the VForce reeds I found that the sled had a slight hesitation when driving at low speeds. This was caused by a slight lean condition caused be the additional air that was flowing through the VForce reeds. To test, I put the choke on the low setting and drove around a bit. The low setting of the choke added extra fuel to the mix and it cleaned up the problem. To fix this, I turned the mixture screws out about half turn and the motor ran like a top again.

I found this upgrade to be very worth while. It’s not very hard and it helps with the efficiency of the motor. It's too early in the season to tell yet if I'm getting better gas mileage or if I can pull a few extra sled lengths off my buddies, but it's easy to tell that the sled runs better over the entire rev range.