mercury 50hp red band misfire

hello I have a problem with my mercury 50hp 3 cylinder outboard believed to be of 1989 origin (red band) the only identification numbers I can find on the unit are as follows serial number 9549343 on transom mount engine bracket.number on cylinder head reads 1025-8363, 69554/us,pat,3773010, address marine power Europe,division of Brunswick,sa, made in Belgium,vivieres 84822. bad misfire on startup,removed plugs to check,running only on 2 cylinders sometimes number 1 plug sooted up 2+3 dry sometimes 2+3 sooted up number 1 plug dry.tried adding 16 fluid ounces of sea foam to 1 imperial gallon of petrol had no effect just loads of smoke.cleaned out carburettors,changed plugs for new ones,changed plug leads and cnnections,replaced 3 new coils,new ignition switch box,new rectifier (the diamond shaped one) new inline fuel filter.replaced 60 amp/hr 12v battery with 2x inline 54 amp/hr batteries pos to pos ,neg to neg as use electric motor as back up and trolling when sea fishing.the units are mounted on a flatacraft force 4 rib.I would greatly appreciate any help you can give me.thanking you in advance. bill.