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Reed Valves and you.... (explained by Dave Beam MXZ 800 of Snowmobile world)

Snowmobile How-To's

 
 
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  #1  
Old 09-25-2007
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Default Reed Valves and you.... (explained by Dave Beam MXZ 800 of Snowmobile world)

There is a small part in the engines of our snowmobiles that probably does more work than any other part. Cycling up and down at over 300 times a second, the reeds in your motor are the key to good performance, fuel economy and clean emissions.

Our example sled for this article is a 2001 MXZ800 Adrenaline with the Rotax 793 Series 3 Twin Engine using Cylinder Reed Induction.

With 3500 miles, the reeds and entire intake system are ready for inspection and general maintenance.

While this might not be the model of sled that you ride, most of the sleds on the market use a very similar system in their cylinder designs.

MAINTENANCE:
(Chapter 2)

Maintenance:

In a two-stroke engine, the action of the piston moving up and down generates and changes the pressure inside the crankcase. In a reed type engine, the reeds are used to open and close the crankcase from outside atmospheric pressure. When the piston moves up towards TDC (Top Dead Center), there is a vacuum generated in the crankcase. This is also when the fresh fuel from the carbs will supply fuel to the crankcase area. The amount of opening during this stage is directly proportional to the crankcase vacuum. When the piston gets to top dead center, the spark plug creates combustion. This pushes the piston down in the cylinder, generating positive pressure in the crankcase. In order to keep this pressure from blowing back out through the carbs, the reeds are shut, sealing off the crankcase from the atmosphere.

In order for our engines to keep the efficiency that was originally designed by the OEM engineers, the reeds must function flawlessly. Due to the high rate of cycling that the reeds are subjected to, they must be inspected and maintained in order for the engine to run at an optimum level.

Once a year, and most likely at the time of pre season maintenance, your reeds should be inspected. Most manufacturers and mechanics will recommend replacing the reed pedals once a year or every 2000 miles. There are 3 kinds of failures most associated with reed engines: loss of tension, cracking and breaking.

Loss of tension:

Under normal operating conditions, most reeds will have a tendency to become weak and loose their ability to return back to the closed position. When in good condition, the reed pedals should rest on the cage with no air gap (image 1-1).

If the reed pedals are sitting more than .020 of an inch off the cage, they are getting weak and will need replacement soon (image 1-2). This will most likely be the condition that will be found when it is time to check them for regular maintenance. Since the reeds do not return back to the reed cage, it takes longer for the reed to seal against the cage when the crankcase pressure increases. This will result in fuel charge wasted and blown right back out into the carbs and into the airbox. This pulse of fuel will disrupt the vacuum of the carbs and cause fuel economy and performance to decrease.

Fortunately, the cost of OEM reeds is typically low and can be done by most sledders with a basic mechanical knowledge.



Cracking and Breaking:

If the reeds become weak and replacement is put off too long, they can eventually begin to fatigue and start to crack or actually begin to break off in pieces. If a reed breaks it can be pulled into the engine and cause damage to the internal parts. If a reed is cracked it will not seal, which will lead to the positive pressure that is generated in the crankcase spilling large amounts of fuel back into the carbs and airbox area. Since a large amount of fuel is being lost through the carbs, the engine can actually start to run lean and could result in serious engine damage.

A cracked or broken reed can be the reason that an engine is hard to start, is getting poor fuel economy or backfires frequently.

The top picture shows you a reed cage that is not sealing proporly on the bottom side...

Pic 2 shows the way the reeds should look if they are proporly sealing against the cage.

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Old 09-25-2007
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Default Re: Reed Valves and you.... (explained by Dave Beam MXZ 800 of Snowmobile world)

In this section we will discuss the procedure to disassemble the air box and carburetors so we can get to the reeds. This is a straight forward procedure, but if you have any doubt in your mechanical ability, you may want to talk to your preferred service center or dealer about this kind of service.

*Please note that this procedure is featured for a 2001 MXZ800 equipped with the DPM feature. Most sleds will be very similar to the following example but basic common sense should be used.

We will begin by removing the airbox from the carburetors. Loosen the screws on the clamps that are between the carburetors and the air box. A long shafted Phillips screwdriver will greatly help you to reach the clamp on the far carburetor.

Next unplug the airbox temperature sensor. *Note: Only Ski Doo DPM equipped models will require this. Remove all the vacuum lines and wiring from the DPM manifold and label them. Then remove the Manifold from the sled. *Note: Only Ski Doo DPM equipped models will require this.

Once the airbox is free from the carbs, remove the airbox from the sled. Loosen the Phillips screws on the clamps that are holding the carbs to the manifolds. There's no need to remove the cables or the fuel lines, they should be long enough to get the carbs out of the way.

Next using a set of pliers, squeeze the clamps that hold the balance tube (large black tube over the manifolds). Then using a 10mm socket, remove the bolts that hold the manifold to the cylinder. Be careful, the threads are delicate!!

Once all of the bolts have been removed, gently pull on the manifold and remove the cage from the cylinder. Try not to knock any dirt or other contaminants into the cylinder. Plug the cylinder with clean rags or paper towels to prevent contamination. Using a small Phillips screwdriver, loosen the small screws that hold the reed stop on to the cage. Be careful, the screws have Loctite on them and will have a tendency to strip the heads.

Once the reed stops have been removed, gently pull on the reed so as to not damage the rubber on the cage. *NOTE There is a notch on one corner of the reed, make sure you note where it is when you take it apart. Inspect the reed cages for signs of wear. The rubber may flake off over time where the reed meets the cage. If there is any rubber missing, replace the cages also.

Take the opportunity, since the manifolds are already out of the engine, to inspect them for any cracks or damage. An air leak here can cause serious engine damage!!!

One you have acquired new reeds from your dealer, reassemble everything in reverse order. The screws that hold the reed stops and reeds to the cage will require use of a thread lock such as Loctite Blue. Make sure that all gasket surfaces are clean and that there are no air leaks between the manifolds and the carbs.

Reattach the wiring and vacuum lines for the DPM System and your done. Since the carbs are detached from the engine, it might also be a great time to inspect the carbs and clean them.


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Default Re: Reed Valves and you.... (explained by Dave Beam MXZ 800 of Snowmobile world)

I have the rest of the pics but I have no idea why they wont post in the right order.... the send 6 and 8 up to the top.... I cant figure this out... Plus they take up WAY WAY TO MUCH SPACE.... Any help here?
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Old 09-25-2007
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Default Re: Reed Valves and you.... (explained by Dave Beam MXZ 800 of Snowmobile world)

I chopped the shit out of it, sorry. Something was goofy with the attachments system for a bit there I think.

Try again. If you still have problems, let me know.

I know the upload tool blows donkey cack. It's actually easier to open 2 windows both on this site, and upload the pics into the gallery and use [img] tags, like explained here:

http://www.slednutz.com/Forums/index.php?topic=36.0
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Old 10-05-2007
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Default Reed Valves and you.... (explained by Dave Beam MXZ 800 of Snowmobile world)

Hey you wanna finish this one up or leave us all hanging?
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Old 10-18-2007
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Default Reed Valves and you.... (explained by Dave Beam MXZ 800 of Snowmobile world)

Senor Douche ....
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Old 10-18-2007
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Default Reed Valves and you.... (explained by Dave Beam MXZ 800 of Snowmobile world)

Quote:
Originally Posted by Ibreakstuff
Hey you wanna finish this one up or leave us all hanging?
ill get to it later today
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