Check for a intake leak around the manifold base 318's are good for that. Also the carb was designed for spark emission control. It is too lean and I would assume does not have a O2 sensor in exhaust. Get another 2 bbl I have several but I never had a real problem with rebuilding them. Stay away from a 71 model it is a rochester design hard to get parts. By the way you did check the heat passage in manifold they love to plug up