The float needle cannot be seating properly in the needle seat,either that or the float is badly adjusted or the float may have a split in it which would stop it being bouyant and pshing the float needle into the seat.im sure thse derbi sendas have a vaccum tap on them ?if so then the fuel tap is also faulty ,check to see if it has a vaccum tap and if so check toi see if the fuel flows out of the fuel hose without it running.also i ahve known the fuel taps to break inside and the fuel runs down the vaccum hose into the inlet manifold and then simply fills the crankase with fuel,this will of course flood the engine badly and foul the spark plug whcih will stop it sparking and starting very easily,hope this has helped you out.
adam bike doctor(of milton keynes motorcycle centre), January 2011
