Troubleshooting a Two-Stroke Engine Reed Valve

Written by tom lutzenberger
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Troubleshooting a Two-Stroke Engine Reed Valve
Reed valves can be used with good results on two-stroke engines. (motocross image by Jerome Dancette from

Reed valves on a two-stroke engine tend to be somewhat temperamental, as the inner workings of the assembly are very sensitive. If the parts are damaged or worn down, they can immediately impact the air/fuel mixture from the carburettor, which then damages engine performance. However, reed valves are often worth the time it takes to troubleshoot, since the design cuts off backflow and makes carburetion more efficient in a two-stroke engine. Solving many of the problems with reed valves includes eliminating other issues that could cause failures.

Skill level:

Things you need

  • Screwdriver
  • Crescent wrench
  • Shop rag
  • Socket wrench and sockets

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  1. 1

    Turn on the motorcycle or scooter engine and get it running, if possible. Wait about five minutes for the engine to warm up. Pull on the throttle to see if the carburettor responds correctly and quickly. Turn the engine off. Turn the fuel supply off.

  2. 2

    Use a screwdriver to disconnect the banjo clamp holding the carburettor to the intake hose connecting to the reed valve manifold on the engine. Pull the carburettor free and disconnect the fuel line connected to the side of it. Use a crescent wrench to disconnect the fuel line.

  3. 3

    Hold the end of the fuel line in your hand with a shop rag and slowly turn the fuel supply on to see if fuel is flowing properly. Turn it off when fuel begins to spill out onto the rag.

  4. 4

    Place the carburettor and fuel line aside. Unscrew the banjo bolt holding the intake hose to the reed valve manifold. Put the hose and clamps aside. Look inside the reed valve manifold to spot any signs of damage to the internal reed petals. Use a socket wrench and sockets to remove the manifold from the engine intake.

  5. 5

    Pull the reed valve manifold apart by hand once it is loose from the engine. Take out the old reed petals and replace them with a new set. Re-bolt the reassembled manifold to the engine again with the socket wrench. Screw on the intake hose clamp after reinstalling the hose.

  6. 6

    Reconnect the fuel line to the carburettor and reinstall the carburettor to the intake hose. Tighten the securing clamp on the hose locking the carburettor in place. Turn the fuel flow on again. Turn the engine on again and try out the response. Take the motorcycle or scooter for a ride to test performance.

Tips and warnings

  • Checking the spark plug colour in your cylinder using a socket wrench can tell you quickly if the engine is running with not enough fuel (white) or too much fuel (oily black). A proper fuel/air mixture should result in a proper spark plug tip colour when operating correctly (chocolate brown).
  • Do not have any flammable sources nearby when working on the carburettor or fuel line of the engine. Gas fumes can ignite easily.

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