can you tell me why my petrol Nissan Almera Tino cuts out when started ?

When starting my Nissan Almera I have to pump the throttle & rev excessively to get it running properly. If I do not rev it the engine floods (strong smell of petrol) & it will not start again for a long time. This is worse when cold but can be the same when hot except the fault takes a shorter time to clear (perhaps the fuel evaporates quicker when the engine is hot).