Harris=aerox

I de restricted the bike and added in new rollers ( 3.3g ) and also added a stage 6 pro replica exhaust, it trys to reach 30mph but cant and only reaches it goes down hills, but when it does reach 30mph the stage 6 exhaust kicks in and it flys right up to 50mph. I've had this bike for a year but it was sitting outside my house for 6 months of that year and ive only really been riding it 4 months. I fitted these things after i went 500miles and they were fitted the other day. Can anyone tell me whats wrong with it and why it isnt getting up to 30mph ???? PLEASE HELP Is it the spark plug ?