2001 Mercury 90 3-cyl rough idle

[dspence]

Hi All,
i have a 2001 Mercury 90 3-cyl, which runs perfect at 1/4 to full throttle, but always idles very roughly, as if occasionally misfiring - occasional drop in rpm with the engine kicking and bucking a little.

Has new plugs, idle timing is correct. All cylinders spark (how can I tell if they occasionally are missing? I see no obvious missing on a timing light).

All idle screws on the carbs are set to 1 1/2 turns. I tried tweaking these screws. Fully closing the screw on the top carb makes absolutely no difference, opening it too far makes it run rougher. Over-opening the screws on cyl 2 or 3 makes it rev faster, closing too much bogs the engine.

Interestingly, the engine runs absolutely no differently if just spark plug 1 is disconnected, or if just spark plug 2 is disconnected, but it stalls if just 3 is disconnected. Not sure if this is normal or not.

Old plugs didn't seem too bad, perhaps plug on cyl one was wet?

Any ideas or suggestions for tests that people had would be appreciated,

thanks,
daev

 

[StuartT]

Re: 2001 Mercury 90 3-cyl rough idle

Amazingly you are starting a new thread which could fool me into thinking I wrote it, and if you refer to a my current thread "Idle Air Screw - No Effect", you will see why. Take advantage of what others have contributed to the exact same problem.