Home Tips and Tricks How Things Work How A 2-Stroke Reed Valve Works

How A 2-Stroke Reed Valve Works

How A 2-Stroke Reed Valve Works
How A 2-Stroke Reed Valve Works

This Is How The Reed Valve Works In Your 2-Stroke Dirt Bike

How A 2-Stroke Reed Valve Works

The reed valve is easy to overlook during routine maintenance, and under-appreciated when it comes to performance.

In this article, I’m going to show you not only how it works, but also why it must function correctly.

To help demonstrate this, I built this air chamber out of plastic containers.  The 1st chamber is the carburetor, and the second chamber is the crankcase with the reed valve in between.  And for my engine, I’m using a vacuum.

Engine Analog Vacuum

Reed Valve Construction

Most reed valves are a simple design, and most are build the same way.  There will be a frame, petals, and in this case, a guard.  The pedals are made from various materials, depending on the age, make and model of the engine.Cage Over Petals Opening With Spacer Reed Valve Opening Reed Valve Petals

Steel, fiberglass, and carbon fiber are the most common.  The way the petals are secured varies, but if screws secure the petals, use thread lock!

Even the smallest of screws can wreak havoc on your crankcase!

Related: If you put a hole in your crankcase cover you moght be able to fix it with some JB Weld.

How A Reed Valve Operates

A reed valve is simply a one-way valve. The petals open during vacuum to allow the air-fuel mixture from the carburetor into the crankcase during the up-stroke of the piston. It will then close under pressure during the down-stroke to allow the mixture to be forced up into the cylinder.

Reeds Opening During Piston Upstroke

But it does much more than that.  The reeds also turbulate the incoming air-fuel mixture to atomize the fuel as much as possible before entering the combustion chamber.

Better fuel atomization means more efficient use of fuel, meaning more power.

What Happens When Your Valve Doesn’t Work Correctly?

When your reed valve is damaged or worn out, the air-fuel mixture can not stay in the crankcase when under pressure on the down-stroke.Reeds Close During Down Stroke This will result in less fuel entering the cylinder, causing power loss, and backflow into the carburetor, causing numerous problems.

(Learn how your 2-stroke carburetor works here)

Aftermarket Valve Assemblies

Many manufacturers offer reed valve upgrades.  The list is too long for this video, but if you want to go shopping, you can always find a great deal through Amazon!

If you have any questions or anything to add, please leave them in the comments or on our FaceBook page!

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