A novel geometry for enhancing the sensitivity of intensity modulated refractometric fiber optic sensor for detection of adulteration level in diesel by kerosene is proposed. In this multimode plastic optical fiber is uncladded for specific length and bent into S shape. This geometry is simulated and analyzed using Beam Propagation Method in Beam prop RSOFT software. When sensor is immersed in the diesel then diesel acts as a cladding for uncladded S shape portion of optical fiber. As diesel is adulterated by different volumes of kerosene, refractive index of cladding changes which in turn affects the output intensity of the sensor. The investigation revealed that when such structure of sensor is used to detect the adulteration level in diesel then sensitivity gets improved 4 times for two fold increase in bend curvature of fiber. Thus it is highly sensitive mechanism to determine on line adulteration of diesel and also can be used for other applications.