To catch up.....Boat runs 74MPH top speed on GPS. Port Engine runs 4,900 and starboard runs 4,600rpm by the tachs. when gassing the throttle in neutral from idle to approx. 2,000rpm and back real quick the engine pops/backfires out the exhaust when almost back to idle. any thoughts?

Any thoughts?

Reving in N on the water hose or in the water by THROTTLE ONLY only will set the overspeed code hence drops ignition but should set off the warning beeps to - so is your warning horn active while this backfiring is taking place and or throttling in persay N . Your test here with the back firing - does this happen when you are truly driving the boat on the water in gear running forward - throttling up and back - ( accelerating and deceleration running in gear) ?

Overspeed usually sets at 2500 rpms give or take a few here when throttle up in N (persay) - I say persay because it really is called reving in throttle only .

I had a crankshaft position sensor that went south, but was throwing no codes. It was causing RPM loss and weird issues that I thought was a fuel problem. I did a LOT of troubleshooting like fuel/pressure/quality/filters/boost pump; and spark plugs/wire/spark test/misfire tests...
...but BUP aka John helped me narrow it down to the CrankPS.

BUP will know for sure, if you can provide your serial numbers (if you have them), but I have also had to replace my fuel boost pumps due to bad fuel issues (several years earlier than aforementioned problem). I bought fuel in FL that was was labelled and advertised as non-ethanol; but turns out it was after we tested it. I left it sit in the tanks (untreated) for a few months in Florida humidity and it ruined inline filters, f/w separators, and fuel boost pumps on both of my 496s when I fired up the boat for the season back in Virginia.

Thanks for the post Brian. Will add why I posted in post # 23 that response was because in post 21 OP says running at 73 mph plus has some rpm differences from one motor to the next as that can be anything for the cause of that. It could be prop related, engine related problem, sensor related problem, fuel related problem, and including is the engine throttle plate able to achieve wide open position. Anyways his posts ONLY states "while gassing in N " never any mention of back firing while running. Just saying

IMO if a clear description of running issues and a list in order of things that has been checked along with parts changed out, sure would help to determine what the real issue is. Also OP mention from the scan motor(s) are showing 100% available power or am I not correct about this ?.

If all possible post your scan report. I would think this boat is put to bed being in Michigan but the engine still could be scanned during this timeframe without running the engines. . Again just saying and sticking with it, trying to help.

This would be stabbing the throttle quickly in N in the water no alarms when doing this. Timing of alarms does not seem to be related and while just cruising across the lake. The other engine does not do this pop/backfire. It's not real loud as I guess it doesn't matter my true concern is dealing with alarms practically every time I go out. Sometimes scan shows something and sometimes not. I have throttled back, not turned the engine off, scan right there on the spot and show no codes. I have a whole host of replacements and the boat spent the winter with boat mechanic. It was me that requested the boat back for the beginning of boating season here in MI prior to the icebergs being gone from Hubbard lake, so he had no chance to water test. I would have to look but believe it was the crank sensor, water pressure sensors and plugs, batteries, alternator, spark plugs, compression tests, got an alarm in there for an iMac and it was bad and replaced that out on the lake last summer as I keep them in stock, low pressure fuel pump, filters, and then some. I have not checked fuel pressure through rim range under load and need to do that.

There was a CPS replaced last winter. I belive it was on this engine. I will look up paper work when I get home. On the engine that I have the lower rpms that have had the fault codes of OVEREV and FUEL PUMP RELAY CRK, the burden portion of the wiring harness that was repair (? There's a lot of wiring harness that is covered and and cannot get to or see), has the popping sound at when coming down in RPM off a quick stab, I have replaced CPS (TO VERIFY), fuel pump relays and checked all connections there of, replaced alternator, batteries, fuel filters, fuel pump (I guess low pressure or prime), spark plugs, taken compression checks, visual check of spark plug wires, rebuilt sea pumps, replace pressure sensors and plugs, I burn ethanol free gas, treat it and use Lucas fuel treatment, replaced IAC mufflers, checked that throttle cables open throttle all the way to WOT, removed drives and serviced them, replaced gimbal bearings, aligned engines, replaced exhaust manifolbs and risers on both engines, had an IAC alarm this past summer and replaced that. Thanks for the continued help.

No back firing while runwind that I am aware of. Just this pop from one engine and not the other. I will get better info of what I have done and checked when i can get back home to my notes. Boat is 1.5 hours away locked in storage. Thanks again for the help.

Remember, you have a CAM position sensor and a CRANK position sensor.
Cam on front.
Crank on back.

You could also have a prop issue causing RPM difference.
They could not be exactly the same (did you replace one?) or you could have damage that is affecting one's performance vs. the other.

Will add, the lingo from Merc and Merc techs - just in case and any written work orders from the service shop of you need to check out what sensor was really replaced.

CPS = CRANK POSITION SENSOR

CMP = CAMSHAFT POSITION SENSOR