Abstract: Disclosed herein is a class of gas carburettor (9) for low calorific value gas for A/F ratio control to be used engines operating on producer gas with a load following capabilities. The gas carburettor has A/F ratio from 1.2:1 for producer gas to about 5:1 for biogas. The gas carburettor of the present invention has right A/F control at varying load conditions due to its optimized dimensions at the throat (7) and area ration control flapper (6) along with the zero pressure regulators. Additionally, using a by-pass facility starting the engine is established. By the use of a zero pressure regulator on the gas line, the gas pressure in the air and gas lines are maintained nearly the same downstream of the carburettor which helps in managing the appropriate air-to-fuel ratio based on the gas and air passage areas.