aprilia sr 50 cutting out?

it sounds like a weak mixture or some type of fuel starvation,there may be water in the fuel which is obviously not goinjg to burn like petrol will or it may be a blocked jet in the carb or even a faulty vaccum tap.does the bike have a standard air filter and exhaust ? and is the air filter fitted properly and is there any leaks or holes in it?.

adam bike doctor(of milton keynes motorcycle centre)
February 2011