I have a 1981 Ford F-150 with 300ci inline 6, carb. recently it started missing badly at idle and low speeds. It has gotten worse and now missing at all speeds. I have replaced spark plugs and wires and fuel filter / air filter. Old plugs were sooty black indicating rich, same with tail pipe, sooty black. auto choke seems to be opening ok. Would a plugged Catalytic converter cause these symptoms, or what do list members think I should try next? Also I put in some dry heet in case I had water in fuel.