fisherman1
New member
Recently purchased a 2019 used 9.9 4 stroke merc. Took it out for the first time yesterday and noticed a bad vibration at trolling speed and when accelerating. Almost felt like one cylinder not firing. Pulled the plugs and one plug looked fouled( insulator was black). I'm new to these motors but they surely can't run like this. The prop spins straight, only 1 small mark on it. Engine has 10 hrs. Any insight?