An engine starts heating if it has not been properly serviced from time to time. Servicing includes checking of its engine oil level, which is the major cause of heating, when available in low amount. Engine oil lets the engine work smoothly in harsh environments and its low amount is a clear indication that friction inside the components will increase and therefore more heat will be generated. Small amount of heating is expected as the engine of Bajaj Discover 125 is air cooled only, but higher heating points directly at the level of fluids inside the bikes running mechanism. You can contact your nearest service center before the parts of the engine gets seriously damaged and the engine seizes itself. The regular service interval of Bajaj bikes is 5,000 kms and one should strictly get the bike serviced within the period for better engine life and mileage.