Give me the name of the Spark checkers you use ,,,please .All I see are clear plastic as a shield, which wouldn't work for long

Did you ever find the problem?

Coil change-out - same
Swapped #6 and #4 wire to see if plug discoloration (not fouled but browner) - stayed at #6
Checked valve spring preload - all on that head right on spec
Cylinder leak-down test - 8 - 9% and consistent cyl to cyl
Cylinder compression test - all good and consistent
Fuel injectors flowed and tested for pattern and leakage at Kennedy's dyno-tune - all four injectors on that side were perfect

Boat is on the computer to see if any of the sensors from starboard motor are significantly different than port motor sensors

Thanks, going thru the same problem.

We were chasing a 525 issue we thought was fuel/spark related last year and ended up being a cracked header.

Have no idea what anyones engine serial #'s are but when ever working on marine apps especially stock OEM it is very important to post engine serial # or drive serial # if working on the outdrive for help - parts - possible fixes and parts bulletins along with Service bulletins.

Anyways HP525 EFI engine serial 0M950064 and all below had PCM calibration change. The change help a lot with poor running conditions and heavy deposits especially soot on the transom. Also make sure you do not have a bad IAT = Intake air temp and or CTS = coolant temp sensor and or its wiring along with connections. I would look there as well. Next clean flame arrestor and good amount of air flowing into the engine compartment.

You mentioned soot on your AT. Rich air fuel mixture, causes unburn fuel that exits thru the exhaust - hence sooty transom and poor running conditions. Thought maybe before it was your injectors so I went back in my memory bank for a possible cause / fix. Good luck.

The PCM calibration change went out to dealers and is listed in SB 2004-04 to help anyone that falls under the engine serial listed..

Next not knowing if your boats are a fairly new used boat to you - just bought and or had the drives worked on but make sure the gear ratio's are the same set ups in both outdrives especially if the engine checks out 100% with everything. Just saying seen this about a handful of times in the past couple of years. People just slappin what they can on the boats to get rid of them.

Also what are fuel pressures thru out the rpms ? I don't think it was brought up or least didn't see it mentioned.

Pressure tested headers to 50 for over an hr with no pressure loss.

Flame arresters power washed from inside out per Merc recommendations for belt dust build up.
Same call to Merc I gave them my SNs and they said my PCMs were up to date
The boat is at shop and guy is gonna run the full set of tests to verify TPS, MAP, etc. I'm hoping it's one of those.
Drives are like new. I run stuff hard but they get lots of lovin to keep them healthy as could be.

I run dash analog FP gauges that show 38-39 WOT but I also added the Merc monitor to convert the CAN Bus digital engine info to NMEA 2000 so my Garmin 740S chart plotter can display all the engine parameters from both motors on virtual gauges and that shows them closer to 40-41 WOT. I believe they're 43 lb injectors stock.
What I need to do is switch my Merc Monitor to the Starboard motor because the MM can display codes, hrs, etc but for just one of the two even though they're ganged together on the bus. It can convert both motors to NMEA 2000 but only display one engine codes, engine name, hrs, etc.

I had gotten the suggestion to swap PCMs but in the AT I think you'd have to pull the motors the way they're packed in under the transom.