The Reed Valve

The Reed Valve

The reed valve is an important component in the two-stroke engine. It controls the amount of fuel and air that is admitted to the crankcase during the compression and transfer cycles. Without a reed valve, fuel could be pushed back through the carburetor into the air filter and cause fire hazards or restrict the amount of fuel that passes through the carburetor to the spark plugs. Reed valves solve this problem and also improve power from idle to full throttle.

The design of a good reed valve requires both art and science. It needs to be light, in order to reduce inertia and follow engine revs more effectively; durable, to resist fatigue fractures; and elastic, so that it returns to its closed position quickly after each engine revolution. It can be made from metals, but composite materials such as glass fibre and aramid (Kevlar) fibre are more suitable due to their high modulus of elasticity.

Optimal reed petal thickness and stiffness are selected through trial-and-error, but they significantly influence performance characteristics of a two-stroke engine. Thinner, more flexible reeds offer faster acceleration and lower RPM power output; while thicker, stiffer reeds provide greater engine torque at higher RPMs. Most modern reed cages use four petals per engine, which reduces petal fatigue and improves lifespan.

Reed valves can be made from several different materials including aluminium, bronze, brass and plastic. Aluminium is popular because it offers good strength, corrosion resistance and is very light. However, its softness and sensitivity to vibration can cause damage to the valve seat. Hence, it is often combined with a harder, more resilient material such as carbon fibre.

A reed valve is a mechanically simple device with no moving parts, but it has some very complicated dynamics during operation. The reed petal moves back and forth to open and close the valve, but its movement must be controlled precisely so that it is always at an optimal position for the flow of fuel and air, and it can do this over many millions of cycles without wearing out.

Over time, the screws that fasten the reed body to the engine can loosen (this is usually because they were not tightened correctly at the factory or threadlock applied to prevent them from loosening). When this happens the fuel pump stops working properly and the engine starts to hiccup at high RPMs or shut off completely at full throttle. A common repair is to remove the reed valve body, clean it and apply some RTV sealant to the gasket surface. It is also a good idea to check the small port that goes from the engine side of the reed body to where the carburetor mounts, making sure it is clear of obstructions and free from sealant. It is also a good idea to increase the passageway's diameter to 3.5mm (using a 3.5mm bit) as this will help the fuel pump perform at WOT. This will make a noticeable difference in fuel economy, engine performance and throttle response.