A reed valve lets fuel mixture into a two-stroke crankcase and stops it flowing back out of the carburettor when the piston goes down. If a reed is broken or stuck closed, the bike is very hard to start, or may not start at all. A reed that does not seal can also spit mixture back through the carburettor and leave the engine weak at low revs. Reed valves are normally reliable.
Is it safe to ride?
Ride with care, short distances only. A broken petal piece can enter the engine, so stop and have it checked if you find a missing piece.
Quick checks first
- Is there fuel at the carburettor, and is the spark plug good?
- Does fuel mist or wetness show at the carburettor mouth after kicking?
- Did the problem start suddenly, perhaps with a bang, or build up slowly?
Diagnose it step by step
1Does the engine have a spark, fuel and good compression?
- Not sure: do the basic checks in Engine turns over but will not start first.
- Yes to all: go to step 2.
2Is there fuel mist or wetness coming back out of the carburettor when you kick or crank?
- Yes: the reed may not be sealing. Go to step 4.
- No: go to step 3.
3Are there signs of a leak after the carburettor (cracked rubber, loose bolts)?
- Yes: Find an intake air leak.
- No: go to step 4.
4Remove the carburettor and look at the reeds.
- Petals bent, cracked, chipped or stuck open or closed, or the stop plate loose: Check a reed valve and its petals.
- All fine: have the crankcase seals and compression checked: Test compression.
Causes, most likely first
| Cause | How to confirm | Fix |
|---|---|---|
| Broken or chipped petal | Light shows through, missing piece | Reed valve check |
| Petal stuck closed with gum or rust | Petal does not move or spring back | Reed valve check |
| Loose or bent stop plate | Petal bends too far or flaps | Reed valve check |
| Air leak at the intake | Idle changes with spray | Intake leak test |