what is a reed valve

[Autophysn]

It may be a stupid question, but what does it do? what is it's purpose? What happens if it is bad? How can you tell if it is bad? :wondering

[Brad200X]

Not 100% sure what they actually do, but 2-strokes don't run too well without them, unless you have some sort of forced induction. I believe they regulate the amount of fuel\air that gets to the combustion chamber, based on the RPM. How to tell if they're bad? Broken or cracked is usually a good sign that you need new ones (I've never seen this in my experience). A good way to tell if they're bad is to hold the reed cage up to a light, and see if they have any gap around the edge where it meets the cage. You may also notice some hesitation when riding, assuming that all of the other major parts of the intake\ignition are in good working order, and clean (carb, filter and plug, for example). They also feel more flexible compared to new ones. Are you in the market for new ones? If you are, buy reeds based on your riding style. Stock reeds last longer than carbon, but don't give you the same performance (I think). I just put a set of Boysen reeds in my KX, and I like them. Not quite as good as carbon fiber, but they look beefier. Hope that helped.

[Nick_R_23]

The purpose of a reed valve is just that, a valve. Since 2 strokes dont have physical valves like 4 strokes, reed valves take the place of them. When the piston goes down (sucks air and gas in) the reed valve opens up because of vaccum and lets the air and gas in. When the piston comes back up, the reeds close because of the pressure.

The way to tell if they are bad is if they are cracked (plastic or CF) or like mentioned above, if you hold them to a light and can see through them. (plastic, CF and metal).

-Nick :TrikesOwn

[tecat-z]

In simple terms, a one way valve. Allows air/fuel to be drawn itno crankcase when piston is traveling upwards. This creates a vacuum situation. Then when piston is traveling downwards and compressing charge into transfers and into cylinder the reeds are held against block to create a seal. Otherwise reverse compression would push charge back through carb and into your filter. This is a basic explanation.

[Autophysn]

In simple terms, a one way valve. Allows air/fuel to be drawn itno crankcase when piston is traveling upwards. This creates a vacuum situation. Then when piston is traveling downwards and compressing charge into transfers and into cylinder the reeds are held against block to create a seal. Otherwise reverse compression would push charge back through carb and into your filter. This is a basic explanation.


If they are check valves, then how do they affect performance?
What makes one type any better than the other?

[Jason Hall]

Ed explained It very well. I will add that certain reed valve's Boyesen rad valve, V-force. Change the way the air/fuel reaches the cylinder. A Rad valve helps even out the flow to the cylinder. Because the carbs on certain engine's are angled that can change how evenly the air flows through the Intake ports. V-force reeds speed up the air charge to help give better throttle response. The reeds also help to break the air/fuel mixture down for better atomization. The more reed tip surface the more the air/fuel gets mixed. On the older V-force reeds you could adjust the reed tension tighter looser. The reed tension can help change flow at different engine RPM's, more or less velocity.

[SCEADU]

Learned suptin new!!

I did not realize that reeds shut during compression!!! Makes sense though.