I have a Mercury 140 I/O from about 1982 or 1983. I have just got the boat in the water after a number of repairs. When on Land, the engine seemed to run ok, however when I got the boat to the marina it was idling very rough. I had the marina's mechanic perform a tune-up on the engine and he changed plugs, points and wires, and any in-line filters. There was also a new water separator filter put in earlier this year. I took her out last night and it ran for about 20 minutes at a full range of speeds from idle to full open. After a period of fishing, the engine started up alright and seemed to work alright as I ran it at low throttle for about 15 minutes, when I powered up to full throttle, the engine started to run rough (almost like quick pulses). I throttled back and it was still running rough. Tonight, I filled up the tank and added a bit of octane booster. When I turned on the boat, the idle was a little rough before, and there was a little grey smoke present (similar to before the tune-up). I then went and adjusted the carb barrels (Rochester 2bbl) until they were only opened 1 and a quarter turns. This really made a positive difference in the idle, so I took the boat for a run, after a few minutes, I powered up to full throttle, but about 5 minutes into the run, the roughness returned, I slowed down to around 2000 rpm but there was still the roughness present. <br />The carb was rebuilt last year by a professional shop and things seemed to run fine then. Any suggestions you can provide are very much appreciated as sails are looking better all the time. 
