I have a 1986 Johnson 40 hp with 145 lbs compression in each cylinder and a nice blue spark to both plugs. Engine idles find and works great until I try to get it up to plane. It goes up once in a while, but more consistently seems to bog down and "plow" instead of planing. When I push in the choke while at the highest speed I can go, the engine begins to die. My OB mechanic thought the problem was a bad gasket and so he replaced it. When I tested it that afternoon after he had replaced the gasket, I could get it up on plane. The next day, it bogged down at high speed and will not plane. I am thinking that it is probably the high speed jet or timing advance problem. Someone please help.