How Does a Two Stroke Power Reed Valve Work?

Written by tom lutzenberger
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How Does a Two Stroke Power Reed Valve Work?
Many modern carburettors in high-performance engines use a reed valve. (moteur de moto image by Emmanuel MARZIN from

A reed valve in a two-stroke engine works by allowing the fuel flow to go easily in one direction but not backwards. In doing so, reed valves help prevent backflow of fuel and air from returning to the carburettor, which could cause inefficient combustion.


The two-stroke engine reed valve gets installed after the fuel line and carburettor and before the engine intake port. The assembly involves a frame to which the reed petals are attached; the frame is then bolted above the engine intake port. As the engine pulls fuel and air by vacuum, the valve petals open.

Signs of Failure

Reed valves are only as good as the valve petals used. If the petals get bent or damaged, they cannot close properly to form a good seal. Performance then gets shoddy as the engine starts to flood with too much fuel. With a certain amount of flooding, the spark plug will foul.


Reed valve assemblies are fairly straightforward and can work with a variety of carburettor designs, as long as the carburettor clamps to an intake hose. Bolt-on carburettors that attach to the engine case or a carburettor box, such as on vintage Vespa scooters, cannot use reed valves without serious modification.

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