When I fill it up to brim with petrol the bike will run fine, but only if the fuel tap is in the reserve position. If I put the tap in the on position it coughs and splutters and eventually cuts out after half a mile.

So i have been running the bike with the tap on reserve. After 60 miles or so it starts to cough and splutter again and cuts out. There is still half a tank of fuel in the tank. If i pull in and let the bike have five it will start again no problem and run a few more miles and then cut out. I have to do this ritual all the way to the petrol station.

Once filled up again it will run fine for another 60ish miles on reserve tap.

Motorcycles - Derbi - Cross city - 2008 Derbi Cross city