Mercury F90 2002 - sneezing in mid range
Mercury F90 2002 model
Need some help here!
I’m having an issue where cylinders #2 and #3 are misfiring and “sneezing” in the mid-range, around 2000–3500 RPM. The engine runs perfectly at idle and at full throttle (5500 RPM).
The hesitation in the mid-range makes it harder to get the boat on plane, but once it passes around 3500 RPM it runs perfectly and feels strong.
What I’ve done so far:
Cleaned all carburetors in an ultrasonic cleaner
Set pilot screws to 2 turns out
Synchronized the carbs (helped a bit, but not completely)
Replaced fuel lines, filter, etc.
Running only 98 octane fuel
Replaced all spark plugs (they look good after a full day of running, even with the issue)
Replaced both ignition coils (cylinders 2 & 3 share the same coil, but no change after replacement)
Compression tested all 4 cylinders at 185 psi – all equal
The problem improves when the engine is warm, but there is still some sneezing.
Any ideas what else I should check?