1999 mercruiser 7.4 mpi timing issue

I bought this boat about six months ago. It just doesn't want to go above 3400-3800 rpm. I've had 22p prop 19 p prop, 17 p prop, it planes in about 3 seconds with the 17 p prop but just struggles to get above 3400 rpm. I have verified compression, verified correct spark plug wires in correct firing order, had a fuel pressure gauge on the fuel rail and it is at 43psi all the time. I don't have a reader but using the forumn used a test light and have no codes. I followed procedures shorting the AB to time the boat and it is at 8 degrees and advances to 32 . But when I checked timing on the lake it starts to retard timing after 2000 rpm. When I hit 3200 rpm I am only 21 degrees advanced. I have unplugged the knock sensors no change. I replaced map and throttle sensors no change. I am wondering if it could be the ignition control module. Not sure how to test it. I get automotive parts cheap but the timing curve is different on the automotive ignition control module versus the mercruiser part from what I understand. Following another thread from someone with a similar problem they said clip the white wire in the group of four coming out of the distributor. This would prevent the ecm from retarding timing and go off of the ignition control module for timing. That didn't ,make a difference other than triggering an alarm. So what do I do now. Oh and new spark plugs.

Is the ignition module mounted to the distributor on the outside or under the cap?

I'm pretty sure it's mounted to the outside. (I'm new at this). But after searching for information on how to test this an article mentioned that the fuel injection computer can advance and retard timing. That was an automotive article. I haven't seen anything mentioned like that in the boat forums or is that what the ecm does? I'm just trying to look at anything that controls timing. One more item may or may not be relevant. One day on the lake the tach started showing 600-800 rpm higher for the same speed than before. I have a perfect pass installed and the tach was showing different rpm than the perfect pass. Which I'm baffled by because if I understand it the perfect pass pulls tach info from the boat tach. It only happened once for about an hour then went back to normal.

Ok got back to the boat. The ignition module is part of the distributor. Had to use my phone to get a picture because I can't get my head back there. I realize the timing curve is different using automotive vs mercruiser but I can pick up an automotive module for $40. Thinking this would at least tell me if the current module is bad wouldn't it?

No suggestions?

Well I'll post for someone that may find this thread in the future. I replaced ignition control module and also the coil. When I pulled the coil out you could see some areas that appeared to be arcing. I'm hooefully this was the issue. Pain to get the coil out as you need a 10 mm 12 point socket to undo the bolts. I'll post the results after I run the boat.

Well, partly better, I now get 3800 rpm regularly. But I have much better power after 3000rpm then before I'm still retarding timing when under power at higher rpm. When in neutral I get the full 32 degrees advance. But when under power at wot I only get 18 total advance. I am going to start looking for a short to ground

Gave up, not sure how to proceed. But hey thanks for all the help....not

Unfortunately there are only a couple of contributors on this site who know EFI and EFI related issues.
The rest of us are from the carbureted time.
We all know outdrives and the basic electrical of the boats.
So it is not that no one is answering you it is the few who know are busy fixing boats..............and not spending time on the internet.

Thanks jack. Sorry for the sarcasim. I have just spent so much time working on this stupid boat that I just gave up. It runs good enough and it is beyond my skill set now. From what I can find on the net it leads me to believe that I have a wiring issue that is manifesting itself at higher rpm. Just don't know what to do to narrow it down.