Hey @Fun Times he edited his post and posted SN
Measure fuel pressure at the rail item 18
https://www.mercruiserparts.com/bam/subassembly/31749/11652/90
Your not getting spark
It's not timing, if it was off it would fire or back fire but either way their would be some kind of combustion
Try a cheap spark gap tester and/or run through this test
Merc hasn't changed the coupler splines from Gen 1 thru Gen 2. The shaft is a bit larger and uses a O ring to make a better contact with the gimbal bearing. Bearings also have not changed other then greasable and sealed
My guess is the motor needs to be aligned. Do you have a alignment bar...
Takes fuel, spark and compression for motor to run. State your getting all and motor will not run.
Your not getting one of the 3
If you are getting spark then it's either flooding or bad fuel.
44 to 60 psi is a large swing
Temp will not cause a backfire and should have no issue running cooler
If you have not replaced the distributor cap do that
The coil (Red wire) comes from the Main Power Relay (MPR) item 12, and gets it's power from the Main harness to EFI harness connector item 15 which comes from 50 amp motor breaker
Also note there is a fuse (1 of 3 fuses) that feeds the MPR under a rubber cap next to the relays
I wouldn't reduce pitch, while it will have better hole shot but the rpm will increase and can over rev. If you have an aluminum prop now could look at same pitch stainless
Then we don't know if there is a injector problem or not at high rpm's.
I can take a can of carb cleaner and 12V and test if they open/close and spary cleaner thru them. Need to be tested up to 6000 rpm so they can seen if any are falling off
There "should" be no drop in rpm in or out of gear with the MPI, that's what's the IAC for. What is the reading of the IAC when it's idling at 1000? If its below 40 then it's trying to slow but that's all it has.
Need to compare IAC readings with idle rpm
Above my pay grade but might be something with the Adaptive Speed control @muc
Boat should keep the rpms constant but somehow the PCM goes into la la land when it's needed, but to even get started would need a serial number
