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When Should You Replace Reeds on a 2 Stroke Engine?


2-stroke reed replacement guidelines

Reeds, also known as reed valves or reed petals, are commonly found in high-performance two-stroke racing engines. They improve the engine’s fuel efficiency by helping to control the flow of the fuel/air mixture as it is pumped from the intake tract to the combustion chamber. Over time, these reeds lose integrity and need to be replaced. 

Reed valve petals wear down over time and should be checked every 15 to 20 engine work hours.  If the reed valve petals are chipped, frayed, or no longer create an adequate seal, they should be replaced. There is no standard timeframe to use when changing your reed valve petals. 

Unlike your air filter, which needs to be changed regularly, the reeds should be changed as needed. This will differ depending on how often and how hard you drive. Consistently high RPMs will cause your reed valve petals to wear down faster. Read on to learn more about two-stroke engines, reed valve petals, what they do, and how to know if they should be replaced. 

Reeds Are Crucial and Need Replacing

Combustion requires three key ingredients:

  • A fuel source
  • Oxygen
  • Heat or a spark to ignite the combustion

Without fuel, air, or a working spark plug, your combustion engine will fail to start and run.

In a two-stroke engine, a mixture of fuel and air is compressed into a combustion chamber by a piston. This is called the compression stroke. Inside this combustion chamber, a spark plug ignites the compressed mixture of fuel and air. The energy of this combustion creates pressure, which moves the piston back down; this is called the power stroke. These are the “two strokes.”

When the piston rises in its cylinder during the compression stroke, it creates a vacuum beneath it by decreasing the pressure in the crankcase. This vacuum creates a suction that pulls a fresh mixture of fuel and air from the carburetor. During the power stroke, the pressure in the crankcase increases, driving the fresh mix of fuel and air into the combustion chamber. 

Before the introduction of reed valves to the two-stroke engine design, some of this mixture of air and fuel was pushed back into the carburetor during the compression stage. Now, reed valves located between the carburetor and the crankcase prevent this loss of power by helping to ensure that the air flows in only one direction.

How the Reed Valve Works

This video from YouTube gives a great rundown on the function of Reed Valves:

Two Stroke Reed Valve Explained in Detail

The reed valve petals are the part of the reed valve that sits between the carburetor and the engine’s crankcase. In addition to the reed valve petals, the reed valve also consists of a wedge-shaped block (the reed block) that the reed valve petals are attached to, as well as some sort of inhibitor that prevents the reed valve petals from opening too much. 

During the compression stroke, the low pressure of the crankcase causes the reed valve petals to open, allowing the mixture of air and fuel to flow into the crankcase. During the power stroke, the increased pressure closes the reed valve petals, preventing the fuel/air mixture from flowing back into the carburetor. 

The reed valve petals are opening and closing with each cycle of the engine, at a nearly one-to-one ratio. Consider that if your engine is turning at 5000 RPM, then your reed valve petals are slapping closed against the reed block almost 5000 times per minute. This is what decreases the lifespan of your reed valve petals. 

Sources: CompoTec; Motocross Action Magazine; University of Calgary; Cycle World 

Signs Your Reed Valve Petals Need to Be Replaced

Without visually inspecting your reed valve petals, here are a few signs and symptoms that you might have reed valve petal wear or damage:

  • Difficulty starting the engine – Damaged reed valve petals can lead to less air being trapped in the cylinder, making it difficult for the engine to get its initial combustion when kickstarting.
  • Rough idling – This is a similar problem to what causes the engine to have a difficult start. Cracked reed valve petals may not open all the way, preventing oxygen from entering the engine to mix with the fuel. This will make the engine sound like it’s ready to die when idling.
  • Difficulty with acceleration – Failing reed valve petals can cause the engine not to immediately respond when giving it more fuel and then lurch forward when the fuel is finally ignited.
  • Power reduction – If your fuel/air mixture is being lost back into the carburetor rather than being pumped into the combustion chamber, you will lose fuel efficiency and power. 

These symptoms can, however, be caused by other problems. To know for sure if your reed valve petals need replacing, you will have to visually inspect them. Visual damage and wear to look for include:

  • Cracks
  • Chipping (usually around the edges)
  • Fraying (curled edges)
  • Large or inconsistent gaps between closed reed petal and reed block 

How to Check the Gap Between Your Reed Petal and Block

A handy way to check the seal between your reed valve petals and the reed block is to use a light. Direct the reed valve cone (the pointy part of the reed block) toward a light source and then look through the reed valve toward the light source. If no light can be seen, the seal is good

However, a little light isn’t necessarily a problem. Reed valve petals that are straight and not pre-curved can have a gap up to about .2 mm without the need for alarm. Any more than this gap, and you should replace your reed valve petals.

A Problem You Won’t See

Even if you don’t have any chips, frays, or cracks, and the gap between your reed valve petal and the block is optimal, you may still need to replace the reed valve petals. In addition to closing against the block thousands of times within a minute, the petals are also being forced open thousands of times. This will cause them to lose their elasticity over time. 

So, if you can’t see a problem with your petals and you’ve eliminated any other cause for engine troubles, change out those reed valve petals and see if that helps. 

Sources: Rocky Mountain ATC/MC; It Still Runs; Southwest Airsports; Fix Your Dirtbike.

Material Matters

Older reed petals were cut from stainless steel. They were inexpensive, resilient, and durable. The major drawback was that any chips that separated from the reed petals could cause engine damage. Most reed valve petals on the market now are made of composite materials, making this unlikely to be an issue.

It bears mentioning, however, since you might come across stainless steel reed valve petals. While reed valve petals are available to purchase on the market, there are engine enthusiasts who choose to cut their reed petals out of stainless steel, glass fiber (fiberglass), or carbon fiber.

Source: TorqSoft

Conclusion

While there may be no set schedule for you to replace your reed valve petals, they should be maintained and checked every 15 to 20 engine work hours to ensure that they are working correctly and not damaged. 

You should also check your reed valve petals if you experience trouble with starting the engine or its performance while running. Don’t forget, this will not necessarily be an easily visible problem, so make sure you use the method above to take a closer look, especially before you risk a breakdown of your engine.

Teddy Henderson

Teddy is always fiddling with small engines, picking up thrown-out string trimmers or tearing apart dirt bikes. He shares what he learns along the way. Hopefully, you'll have less headaches than he has had by learning from his mistakes.

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