NRG Power running really bad after fitting stage 6 pro replica exhaust? please help

Hello, I have a 2010 NRG Power with stage 6 70 kit, running a 21mm TNT carb and 4g rollers. I changed the exhaust (wasnt standard but definitely not a performance one) to a stage 6 pro replica exhaust and now it just doesn't run right at all.

Very slow from take off to about 5000rpm, followed by a surge of power mid range accelerating fast and revving its tits off for about 3 seconds, then suddenly drops down again and sits at just under 8000rpm at around 30mph. Before it would uniformly rev up to around 9000rpm with good acceleration.

Im not the savviest with scooters so any help would be much appreciated, any ideas people?

 

good luck with it mate I had same propblem with speedfight2 but getting rid of it now for an aerox so its gonna be somone else problem!