petrol smell through exhaust and fuel efficency fell down
 

recently my cbr250r got a starting problem,service center guys checked everything ,cleaned fuel filter and atlast solved problem by changing the whole key set becausse zener diode connection lost.but now i am getting fuel smell through exhaust and fuel efficency fell down drastically.
what may be problem??
o2 sensor
or
fuel injector
or
spark plug
or
engine components ???

Fuel pressure regulator?
I'm not that familiar with the 250....