Its the plastic cone that directs the flow of fuel that wears out, common fault on the Steamers.
You can get a Pingel fuel tap, but pricey at around £90.
Alternatively, you can use one of these for around £30 delivered, from Ebay USA.
YAMAHA WR400F WR425F WR450 FUEL PETCOCK GAS TANK SWITCH
Me and a couple of other Steamer owners are using this with no problems.
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