if what bdcs says doesnt work then you will need to strip the carb and clean it and the jets with some carb cleaner (about 4 quid from your local garage).... if that doesnt work... or you cant be bothered with the mither than a new carb is £30 (this is the option i would go for)... it's a piece of slash to change and takes 20 mins to do and if you want then you can ring me to talk you through it... i can 99 percent guarauntee this will have it running like new again 07871 242012
PS, while your changing the carb you may aswell do the filter, again a piece if slash