92 Mercury 3 cyl Outboard loss of power

[quantum900]

We have this motor on our 24' pontoon. Last season at full throttle I know we were getting up to ~5k rpm, now it will not go past ~3.5k and just has no power. It seems to run fine, smooth etc but just no power. We replaced the spark plugs and put carb cleaner in the gas, but has not gotten better. I am wondering if one of the cylinders is not functioning. I read that you could pull the spark plug cable while the engine is running to see if there is a noticeable decline in RPM's to determine if or which cyl may not be working. Does one need to wear rubber gloves when doing this? I am wary of touching a spark plug cable in use.

Any suggestions would be greatly appreciated.