JAYANTA BASU A smoke-belching three-wheeler on a city road Calcutta's air will continue to be polluted by the exhaust of old autorickshaws running on adulterated fuel with the government putting the brakes on the conversion of two-stroke three-wheelers to LPG. After advocating conversion for five years, the government found "demerits' in the process. It has adopted a two-pronged strategy of replacing old two-stroke autos with four-stroke ones and penalising the thousands of illegal three-wheelers.