I have a 2000 90 hp merc. 4 stroke that is running rough off idle and stalling. It idles smoothly but as soon as you advance the throttle it stumbles and dies. I have put new plugs in, new fuel filter, bulb and fuel line, checked compression(+100 per cyl), checked carb. sync., made sure all cyl. had spark, and checked fuel pump pressure at both pumps. It also seems as if the #1 and #3 cyl. are running a little leaner than #2 and #4 from checking the spark plugs. I did notice that when the sync. plug for #3 cyl was removed to hookup the vac. gage it would cough and the engine would die, all other cyl. would not. any suggestions would be greatly appreciated.
