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Ensure proper performance and consistent horsepower from your sled year-after-year.

What does this article cover:

Boyesen has assembled a basic pre-season troubleshooting guide that has been written to help you better understand what areas of your reed valve system may be contributing to improper operating performance. This article, although simply written, is based on years of research and development experience pertaining to 2-Stroke snowmobile engines. It is provided as a free service to the Doo Talk community.

Who is this guide written for:

This pre-season guide is primarily written to help owners ofSki-Doo ZX, REV, XP, XP-R snowmobiles. However, the troubleshooting and maintenance procedures contained in this article are also generally applicable to all 2-stroke powered snowmobile engines.


Your sled's reed valve system is one of the more important areas to include with your pre-season maintenance schedule. Your sled's reed valve and air boot are what prepares and feeds the air/fuel required for combustion into your sled's cylinder heads. Hard starting, horsepower loss, improper fuel/air ratio readings, and inconsistent engine performance are commonly associated with improperly setup and/or degraded reed valve system components. Determining what causes your sled's engine to perform poorly involves identifying what is throwing off the fuel/air mixture. Additionally, the ignition timing system and fuel delivery system, if set incorrectly, can also contribute to problematic performance but are not covered within the scope of this article.

Any reed valve or air-boot leaking, or damaged/worn-out reed petals can lead to inefficient airflow and/or problems with your sled's engine's ability to produce consistent acceleration response and peak horsepower. In a worse case scenario a compromised intake system may develop an air leak which is one of the most common conditions to produce engine over-heating and failure. Simply following our pre-season reed valve and air boot system inspection procedure will allow you to discover any compromised components and correct any air/fuel ratio imbalances easily and get your sled's engine running optimally prior to the first snowfall. It may even save your engine from a complete lean condition meltdown.

A common problem experienced by Ski-Doo owners (among other brands) is the degradation of their sled's rubber reed valves and air boots. Two Cycle snowmobiles have not been engineered to accommodate the differing characteristics of some modern fuels. Especially when it involves your sled's reed valve components, use of ethanol- based fuels have been found to speed up, if not cause an array of delamination issues that range from lackluster performance to critical engine failure. Simply stated, the rise in stock reed valve and air boot failures is occurring as the result of a combination of poor quality OEM rubber, alongside the use of ethanol-based fuels. An air leak in your sled's rubber air boot can lead to poor idle quality, engine surging and an overall loss of high rpm horsepower and response from your engine. More importantly, intake boot delamination air leaks WILL cause your engine to run extremely hot, and will most often result in a costly engine seizure. Because delamination air leaks tend to leave no immediate visible traces, this condition causes a large number of engine seizures every year resulting from riders failing to properly inspect their boots. Annual inspection and replacement of your rubber intake boot will greatly reduce the possibility of doing excessive lean-condition damage to your engine.



Engine Symptoms Associated with an Reed Valve Air Leak:

Inconsistent Engine Idling. Engine RPMs surge.

• Hesitation when throttle is opened with a strong, inconsistent engine surge.

• Excessive heat in the exhaust and engine.

Inspection of Your Reed Valve and Air Boot for Delamination and Cracking

1. Gain access to your sled's reed valve assembly.

Remove the airbox, carburetors or throttle bodies, intake manifolds and reed valve assemblies. Refer to your specific model's owner's manual for detailed instructions.

2. Exterior Inspection of Reed Valve and Rubber Intake Boot

External inspection of your reed valve's rubber surfaces is straightforward and does not require you to completely disassemble your sled. To perform the exterior visual inspection of your sled's valve and air boot, look closely for small bubbling on each of their surfaces.

If Reed Valve or Air Boot surfaces bubbles are present:

Surface bubbles often represent the early stages of intake boot delamination and are a good indication that your reed valve or air boot is either developing, or will soon develop internal delamination. If bubbles are present, it is advised that you completely remove the stock reed valves and perform a visual inspection of their internal walls. If bubbles, cracking, or tears are present either externally or internally, Boyesen recommends that you replace your entire valve assembly, eliminating any chance that your sled will suffer from a stock reed valve or air boot delamination issue.

Exterior Delamination Bubbling (A Precursor to failure)

Your reed valve and air boot may be showing similar signs of delamination on their surfaces. This picture shows the subtle bubbling that is the pre-cursor to cracking and "active" delamination. Although not yet contributing to an air-leak, these precursor bubbles indicate that your OEM rubber boot will need to be replaced with a better quality Nitrile boot to ensure that your engine doesn't run with any air leaks.

If Reed Valve or Air Boot surfaces show no bubbling:

Since you have already gained access to your reed valve, it is a simple additional series of steps to inspect the internal condition of your vulcanized reed valve. Completely remove the stock reed valves, taking care not to damage your reed valve's OEM gasket or o-ring surface, and perform a visual inspection of the internal walls of your sled's valves.

3. Interior Inspection of Reed Valve and Rubber Intake Boot

The second precursor to a delamination air-leak is micro-fracturing of the rubber boot surface. This early-stage rubber micro-fracturing is not easily seen without bending the boot or applying pressure to the rubber area with your fingers. While applying pressure to slightly deform the boot, look closely for small cracks in the rubber. If cracking is present Boyesen recommends that you replace your entire valve and air boot assembly.

4. Critical Delamination Tear Inspection

Additionally, interior delamination often occurs in the interior reed valve cavity located closest to the cage and the mounted reed petals. The intersection between the aluminum cage and the rubber boot is referred to as the "vulcanization interface". This area is where internal delamination occurs most frequently and it is the area that is typically first to develop into a critical air leak. When this type of air leak happens it is sometimes a matters of seconds before your sled's will experience a critical engine failure.

Severe Internal Delamination (The Hidden Engine Failure)

Some intake boot failures start INSIDE the walls of the reed cage and work their way to the exterior. This is the most critical type of delamination because the amount of total delaminated surface area is large by the time the failure reaches the external ambient air. Interior delamination, if left unchecked represents the majority of lean-engine failures, producing engine seizures VERY rapidly (within seconds) after the air-leak begins.

Boyesen Rage Cage Solution

All Rage Cage systems ship with our proprietary Nitrile formula rubber boots designed to replace the OEM boots that so often dry rot, crack and delaminate causing air leaks and burn down. Boyesen Rage Cage intake boots are made with a proven rubber material that will last longer than any other OEM or aftermarket boot available.

5. While you are at it… Check your Reed Petals

While you have access to the interior of your reed valve you have the opportunity to inspect the condition of your sled's reed petals. If your sled has gone two years or more without a reed petal inspection the chances are good that they are worn soft, or have edge chipping or micro-fracturing that will reduce the efficiency of how you sled delivers it's power. (Refer to reed petal section later in this article).




An snowmobile's overall engine performance (it's ability to achieve consistent acceleration response and peak horsepower), is directly linked to the performance of it's reed petals. Two-stroke snowmobile engines receive their air/fuel mixtures through intake tracts, or reed valves. The small, flexible reed petals (bolted to the reed cages) are primarily responsible for controlling the inlet of air and fuel by the simple process of opening and closing against the surface of the reed cage. Unfortunately it is very rare for any reed petal to last forever. Over the lifespan of every 2-stroke engine it is very likely that it's reed petals will begin to deform, chip, or experience edge micro-fracturing. Throughout its life span, a reed petal flexes millions of times, and its individual composite fibers start to lose their memory, becoming slow to react in unison with the engine. The petal's modulus of elasticity decreases so that in equal engine conditions the petal will deform and open farther than specified. This condition is termed excessive reed "hang-open" and has been proven to contribute to a rich condition in the engine, poor horsepower output, poor fuel efficiency, and inconsistent operational performance.

Types of Common Reed Petal Failure

A broken reed petal condition happens much more frequently than what most snowmobiler's are aware of. Boyesen recommends checking your reed petals for signs of wear at the start of each season.

Why All Reed Petals Wear out

Because reed petals are responsible for the ongoing regulation of air and fuel into the engine, they are the single-most crucial component within the intake system's delivery process. They are the flexible component of the reed cage that "reacts" to the engine's cycling piston - making them the "Gatekeepers of Power". To regulate the incoming air and fuel in proper ratios reed petals are designed to "reactively pulse" as the engine's pistons cycle through their intake and exhaust strokes. This reed petal pulsation, or "FLEX CYCLE" occurs at a one-to-one ratio in unison with the upward and downward movements of the engine's pistons.

Within each flex-cycle, reed petals are designed to flex open in unison with the piston to allow air and fuel into the engine. The petals then briefly close, precisely "kissing" against the surface of the reed cage to stop the internalized air and fuel from backwashing out of the engine. This process is commonly referred to as "Reed Petal Trapping" and is one of the performance tuning areas that can either positively or negatively affect a reed cage's ability to supply the engine with what it needs to maximize power and acceleration response. In a static state (when the engine is NOT running), the reed petals typically maintain a slight gap between the surface of the reed cage. This gap is normal and does not translate into a problem once the engine is running because the piston-induced back-pressure forces the reed petals to make a positive seal against the surface of the reed cage. Although this natural back-pressure allows the reed petals to achieve proper seal against the reed cage surface, overtime, it may cause soft, over-flexed petals to deform, or experience edge chipping. In fact, it has been seen that many plastic stock reed petals become soft after a month or so of riding and begin to further degenerate quickly thereafter as a result of the engine's back pressure through the system.

When reed petal degeneration occurs, most, if not all reed petal designs, regardless of materials used, are unable to properly trap the air and fuel charges in unison with the engine's pistons. In this regard, failing to eliminate air and fuel "reversion" is a known cause of decreased engine performance and can easily be remedied by changing to a new set of reed petals. In addition to reed petal wear, your sled's reed petals can crack or otherwise degenerate when the engine is revved too high for an extended amount of time. This mechanical problem makes the engine difficult to start and can also exhibit a loss of overall performance and torque. A broken reed petal condition happens much more frequently than what most snowmobiler's are aware of. Aggressive riders may consider switching to carbon fiber reed petals because they resist breaking at high rpm.

Engine Symptoms Associated with Worn, or Damaged Reed Petals:

Loss of horsepower and torque. Engine fails to pull hard at high rpm.

• Hard starting, and inconsistent idling.

• Rich Condition at lo - mid rpm range.

How to Inspect your Sled's Reed Petals for damage or wear.

Performing a quick visual inspection is the ONLY method that is reliable for inspecting your reed petals. When checking your reed petals, look for edge chipping, surface tears, cracking, or a peeling of the surface material.

1. Gain access to your sled's reed valve assembly.

Remove the airbox, carburetors or throttle bodies, intake manifolds and reed valve assemblies. Refer to your specific model's owner's manual for detailed instructions.

2. Inspect the reed cagefor foreign material between the reed petals and its seat. If the assembly's petals are pulled away from the cage, cracked or have rough edges, the petals should be replaced.

Types of Reed Petal Damage

There are different types of reed petal degradation that you should look for. If the petals show edge or surface degeneration, replace them with a new set. When checking your reed petals, look for edge chipping, surface tears, cracking, or a peeling of the surface material.

3. Remove the reed petalsand rinse the reed cage components in clean solvent. If the petals show edge or surface degeneration, replace them with a new set. When checking your reed petals, look for edge chipping, surface tears, cracking, or a peeling of the surface material.

4. Measure Reed Petal Static Gap

Check to see if the reed petals are falling within the "gap" tolerance specification of your manufacturer's owners manual. The petals should be seated against the cage with roughly a .015-inch air gap between the frontal edge of the petal and surface of the reed cage. The tolerance for "gap" varies slightly by manufacturer, and also varies per each snowmobile. Always refer to your owner's manual for your sled's specific reed petal specifications.

5. Re-install the new reed petalsand all required components to the reed valve assembly. Check the clearance between the reed cage with a feeler gauge and compare the clearance to you're sled's specifications. If the clearance is improper, check to see that the unit is assembled properly.

Note:Used reed petals may appear to be in good condition but fail to give the proper clearance due to material elasticity breakdown and softening. In such cases the reed petals should be replaced.

6. Complete Re-Assembly

Using new gaskets (if required), replace the reed cage assembly, carburetor or throttle body on the engine and complete all required re-installation procedures.


Founded in 1972, Boyesen revolutionized the 2-cycle powersports industry with the introduction of the Boyesen Power Reed. Since it's inception Boyesen has set the standard in which all other high-performance components and intake systems are judged. For more than 30 years Boyesen has specialized in the design and manufacture of high-performance parts to the power sports industry. To get more Information about Boyesen's complete line of intake performance products and to access the Boyesen Dealer locations please visit:

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