how can i fix my speedfight 2 50cc?

how can i fix my speedfight 2 50cc l/c 02 plate.
i was riding my ped around and when i slowed down it just cut out and wouldnt start again i checked the spark plug and it works fine.
can someone help please also recently got a giannelli reverse sport exhaust fitted with rollers(not sure on weight) could the carb be blocked and if so how do i sort it out. (my mate put yellow coloured petrol in so used as much as i could and filled up with fresh petrol then used half tank b4 it cut out) thanks 4 any help given