A bit of a grumble this one.
On top of the airbox is a little valve. When i have it connected i have lumpy idle and can hear hissing from the fuel cap. If i disconnect it, it runs fine. I have been told it links to a charcoal filter and is something to do with fuel vapour purging. Does anyone know if it is needed? Should i replace the valve or leave it disconnected? ( while it is connected if you hit it it works ok for a short while)