Mercruiser 4.3 LX probs

[Guest]

I think i have developed a misfire problem on my engine, it seems to run fine from cold at low revs for hours, but run it high speed (boat on the plane) after a couple of minutes the revs drop by 200-300 and the boat develops a heavy vibration from the rear. If i rev the engine out of gear it seems fine, i have checked all the leads visually and removed the distributer which showed no signs of tracking. The engine has done 150 hrs and i have fitted new spark plugs. Have i an ignition problem or is it something more sinister?

 

[ArthurWood]

Had a very similar prob w/my Merc 7.4l a last year. After prop and shaft checked, found electrical prob in Ignition Module. That's a little black 'box' attached to the distrib and connected to the coil. There is a 6-pin connector on the 'box' and this was found to have moisture in it. Recently I had to replace that same 'box' because it finally died. I have 1100hr on my engines, though.