200HP OX66 slow to rev up

[max366]

I have twin 2001 OX66 200HP motors, LX200 and SX200 TXRZ. They have been incredibly reliable but now I have an issue with the port engine. Here are the specifics:
Symptoms
1. Engine won?t rev up as fast as the stbd engine
2. Port engine won?t achieve the normal 5400 RPM or so at WOT- it only hits about 4600 (the stbd engine runs at 5400)
3. Port engine requires the throttle handle to be further forward than the stbd engine to hold the same RPM
4. Fuel consumption for the port engine is about 30% higher than the stbd engine (using the Yamaha fuel mngmt system)
5. When decelerating with the throttle handles pulled back the same, the port engine takes longer to come down in RPM
6. Both engines are smooth at idle and while running
History
1. All low pressure fuel pumps were replaced earlier this summer - both engines
2. Just installed a new O2 sensor in the port engine - the original sensor was fouled but not too badly - no change in performance
3. The boat has a single fuel tank feeding both engines
4. Ring free is always used as is Yamalube
5. Checked the plugs (newly installed in early July) and they looked normal
6. Racor 10 micron filters used with new elements

Any ideas on what is causing this? My thoughts are- throttle positions sensor issue; stuch butterfly plates; VST tank filter issue (but it runs smoothly and is getting lots of fuel)

Thanks in advance for any thoughts.

 

[max366]

Problem solved!

Problem solved!

It turns out the problem was- a bad spark plug. Hard to believe, but one plug was clearly black around the center electrode ceramic while all others were white. I haven't had abad plug in years. Prior to checking plugs, also inspected low pressure fuel pumps (disconnected from block and pump up fuel line bulb ans saw no fuel coming out of the pump) and connectors for O2 sensor. No change in performance. Checked/changed the suspicious plug and the motor ran like new. Hope this might help someone else in the future.