Check your throttle cable routing,lock to lock on steering..see if it makes any difference,also lube throttle cable if it is sticky on twist grip..good look.
If is a fuel injection model then check the aftermarket is still using the oxygen sensor (lamda probe). Otherwise the engine management unit is getting false data. if it uses a carb then an air leak in the rubber parts or in the manifold could cause a lean fuel-air mix and your problems. Also check your fuel filter as it may be restricting the fuel-side of that equation, and then do the usual checklist of carb float height, needles jets etc.throttle cable and grip is completly fine, i spoke to someone and they said the carb probably needs cleaning but i'm not sure