Bike misfiring and spluttering help !!!

Hey i got a mbk nitro, i got it as i cant offord to run a car at the moment and used to love my old aerox, anyway it was bog standard when i got it , ran fine when i drove it, only did a few laps round my street, but anyway i put a stage6 racing 70 kit on and a polini for race 3 exhaust and i put it all on properly as iv done it about 10 times in the past etc now it runs like shit, i put a brand new long reach spark plug in, it miss fires really badly and splutters , it used to only do it when it went to kick in to the high rpm but now it does it as soon as you pull the throttle back i dont have a clue what it is ?

please help x

 

put a colder sparkplug in maby? mine did it and i found it was a loose earth wire mine was located on the starter motor

 

what sizes carb, jets and rollers you running?