one of my 96 502's efi's/mpi at around 3800-4500 will start to cut out like it hitting rev limiter , seem o run fine below that. swaped coils last night but it got dark and that didnt fix it. any ideas. maybe knock sensor or the module in distributor??

With the engine running on a hose or in the water wiggle pull all connections including the injector harness.I am assuming you checked the cap and rotor.It also could be ignition wires but check the connections first and check everything.Start at the front of the engine and work your way back.
How much does the tach move when it does this?If its a a large drop thats usually ignition.If its a gradual drop thats sometimes fuel. You could also do a fuel pressure check thats probably not it but just to rule it out.Also check your neutral safety switch They are most of the time on the shifter cables were they attach to the manifold if bad they will limit engine RPM to 3500 as the ECM thinks the engine is not in gear.

Definitely don't rule out fuel pressure, had one do almost exactly what you describe, bad regulator.

ok so , this am i checked everything and couldnt find anything, i had a knock sensor, so i swaped that and also a ignition modual that is in cap.....still does it. Here's the catch!!!

My other motor is doing it now, same thing.

Both motors have brand new cap, rotor, wires, MAP sensors, all stock never apart. have had it runing about 25 hours this year with no problem what so ever.

I couldnt even pull 5000 rpms out of either motor tonight, and it feels like they are starving for fuel, then rpms drop 1000-or so. Boat ran fine, last week, all i did was add 40 gallons of 89 fuel to fill tank.

I pulled fuel/separators too tonight, no water, fuel smells good.

Seem like it does it more in rough water, but it will do it on glass water too. below 3000 seem to run flawless, get up on plane strong.

yesterday, morning i ran the boat right at 5200 then on way back it started to run funny on one motor....now its both.

Ideas????

I have never worked on that motor if it was a 6.2 would be better versed.But in my opinion you either have bad fuel of a common ground wire.I am leaning toward the wire as fuel would not be worse in rough water vibration.

Okay, you need to check that there is absolutely no voltage traveling between the 2 charging systems. I had a similar thing happen when the battery switches were set to "ALL". There is also a Merc bulletin that says the computers will freak out if the charging systems are joined on a twin engine boat. Make sure there are no voltage cross feeds, grounds are not as big a deal as I've been told.

I do have both battery switches set to "BOTH" i allways have had them on both, and never had a problem. how whould i go about seeing if the voltage is traveling between both. Its just odd to me that i have had no issues with running it on both all year.

the boat is new to me this year but have 25 hours run time and 4 25 mile runs i make running hard most of the time in ocean.

Tomorrow im going to crawl around in engine compartments.

keep ideas coming, so i can check things out, thans guys.

Mine did the same thing, ran it for a year or so then one engine was missing, etc. Someone told me to try setting the switches separately and it cleared up, oddest thing.

did this ever get resolved?
Am kinda having the same problem...motor cuts out like its hitting a rev limiter, but I know its not (4500 rpm).
Tried a larger pitch prop, and it was happensing sooner, at like 4300 rpm.

I have a twin setup too....
So I should check the carging system for the batts? I do have it switched to all as well. Its only happening under WOT as the RPMS are right near the top.

Nothing to do with battery switches......it was the fuel filters inside the vst tanks were mostly blocked with sediment. Took pummps out and cleaned filters and it was fixed.....have run 300gal through since and pulls 5200 all day long. Tooks me like an hour to fix. Merc uses 25micron spin on fuel filters on block and they let small particals through.....but merc says don't run anything samller than 25micron as the fuel flow will be restricted and could run lean.....