Hi I have a gilera runner purejet 50 05 model (old shape) ?

My gilera purejet has a 70 kit 7 gram rollers and a sports exhaust the bike is becoming a serious problem it starts on the button but when I ride it it wants to die I'm losing a lot of power and the exhaust pops it's misfiring badly it has a great spark could it be the injection system ??? Please help