Mercury 40hp Limited RPM and Dying RPM at WOT

Ok .... so any advice appreciated ......

Engine 1990 Mercury 2-Stroke 4Cyl 40hp Classic

Was out last week checking Lobster Creels/Traps and noticed engine RPM only reaching 4000rpm where it would usually be 5000rpm @ WOT

2 days after, again out checking creels ..... Same issue but this time after WOT at 4000rpm for 10 mins
while pulling first trap with engine idling I heard the engine just splutter/dying like the fuel in the tank was low - grabbed the tank and it was still half full ?

Engine was surging from nearly dying to its normal 1000rpm - Decided to head home and throttled-up to WOT and on doing so engine was surging from 2500rpm to 4000rpm. It might sit a 2500rpm with throttle fully forward and then 10-15s later surge up to 4000rpm sit there for about 5-10 seconds and die back to 2500rpm ish again coniniously until I got back on the mooring.
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After the initial issue I assumed a bad plug as looked like I might have been firing on 3 out of the 4 cylinders
New surging issue feels like a fuel delivery problem
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Heres what I've done .....

Both Carbs removed soaked in Cleaner and blasted out with compressed air - no change
Fuel pump replaced with a spare I have - no change
new fuel filter fitted - no change
New Plugs fitted - no change

Have not yet tried with a different fuel tank or fuel line/bulb etc
Have not yet tried new fuel
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Although the performance appears to be fuel related - I'm now not convinced it is, and here's why ...

I have a voltage Regulator fitted in my engine in place of the original rectifier that was fitted, it's the regulator that is suited to the engine and has been giving me a steady 14.3 volts since installing it about 4 years ago. I have a dual battery set-up with 1/2/both charging switch. Both are monitored with digital volt gauges and every time I fire up the engine whether at idle or WOT I usually get a steady 14.3 volts but all I seem to get now is a 0.3v increase on my battery's normal 12.3v so say 12.6v even at WOT ??

Now I tested my stator today in accordance with the merc service manual and it's states that resistance between the green/white wires should be 660 - 715 ohms. I tested at 720 ohms so assume that's ok ?
Reading between the two yellow wires was 0.2 ohms and that is also within limits I think - I've left the manual on the damn boat so can't remember the range they specified but it was within that.

Not sure what to do now ? Could a bad Voltage Regulator cause these issues ? or could my stator be playing up under load/heat causing low top rpm and surging issues. I've noticed the surging issue doesn't start until about 5-10 mins after engine is started and being at WOT. It may happen at lower Rpm too but haven't tested in all scenarios.

All ideas or advice welcomed .

It does sound like your voltage regulator is shot based on the lack of voltage increase at the batteries.

Having said that I can't think of a case where a bad stator caused "surging" - not saying that it's impossible, but I just can't recall that being a symptom - poor running, limited rpms (but fairly steady) - ok, but not a surge.

I would check the tank since that is still on your hit list. A bad/clogged pick-up (in the tank), a jiggered anti-siphon valve or a clogged vent (which would be my number one thing to check - a clogged vent will cause a surge) - would be the first things I would rule out before I started looking at the ignition.

have u taken readings at the stator wires that go into reg? disconnect them and see if they are putting out anything, for mine oem stator was around 100v iirc, mine were yellow wires for a 3cyl 60, yours might be green or sth.. see if there are putting out voltage first..

surging could be heat issues on switchbox or coils although imo boat and bike coils rarely fail..

i had a fuel tank that had a leak and i had similar issues - ran fine till carbs dried up, but 5 mins is way too much unless ur leak is tiny.. can u try swap a tank / hose and see if issue goes away?

u can also change the wires from switchbox to coils on the bad plug and plug wires to cylinders on 2 to see if the problems follows the swap - if so possible switchbox issue, if the problem stays on the same plug possibly coil / plug / plug wires issue, or trigger (very unlikely).. triggers usually work fine out of their ohm spec. one of mine is almost double and no issues..

thanks - was out last night and replaced the tank with another and fresh fuel , have s bulb and line I can try if that doesn't do it but typical fired up the engine idled a bit went to throttle off and hey presto as if by magic - snapped shift / throttle cable so ordered a new one and will resume testing when that arrives

Please let me know what you find. Having the same issue with 98 115hp. Checked Compression, replaced fuel pump, filter, plugs and new gas even cleaned carbs.