I recently picked up a nice old 1974 20HP Merc. It has 110 PSI on both cylinders, and looks very clean and underused. Great spark. I took off the carb, took it all apart and cleaned it, although it was very clean already. Reset the float a little. All "looked" good. Checked the high speed timing by pulling the plugs and hooking up a timing light. It was right on. I can't get it to start cold (I am in S. Florida so cold is relative) unless I give it a shot of gas in the carb. After that it will idle but belches out the carb every now and then and will stall. It will restart easy (no prime necessary), but will spit and cough if I idle it down. I have played with the idle screw adjustment to make it as smooth as possible. When I give it more gas it will run OK and not miss .............only at idle. I ran the great elixir Seafoam through it by putting it in the gas but that did not help. Using Champion L78V plugs that I sand blasted. Any ideas on how to get my 37 year old motor running smooth? Thanks 