2+2 ignition "logic"?

Newb here seeking wisdom.

Have a late 80's Mariner 100 HP on a 16' fiberglass open bow. Runs like snot for a few mins @4000+ RPMs with more power than I need. Then drops down to about 2000 - 2500 RPMs even with WOT like it is fuel starved. Am about 90% sure it is not fuel related. Seems like something electrical warms up and then does not work properly. Took it in to my local marine mechanic as I dont have the time, nor expertise to diagnose. They came back and told me the engine is basically about to blow up and dont bother with it. It starts up, idles fine (for a Merc of that vintage) has tonnes of power so I call BS on the dude. He has steered my wrong on my chainsaw in the past but that is another story.

Have the shop manual and the CDI manual. Will get it out back on the water to properly diagnose with the right DVA setup this weekend. My understanding of these engines is that 2 cyls fire on RPMs below 2000 and then once it hits 2000 RPMs, the high speed stator winding kicks in and starts sparking the other 2 cyls, also at which point the high speed circuit in those two carbs kicks in as well. No low speed circuit on the +2 carbs. Once I learned it was a 2+2 engine it made much more sense. Manuals say it is either the high speed stator or the switch box. Or both. Taking wagers on which one it is!

My question is what cylinders are the 2 vs the +2 on these engines? That meaning which ones kick in when 2000 RPMs are hit? Firing order is 1-3-2-4 with 1 & 2 being opposite and 3 & 4 being 90 degrees from 1 & 2.

Stay tuned.....

That motor has spark on all 4 cylinders ALL the time. The ignition has nothing to do with the 2+2 operation. The 2+2 operation is controlled by the carbs ONLY. The motor idles on the top 2 carbs until 1800 rpm then the bottom 2 kick in. The bottom 2 carbs have an accelerator pump to help the transition the top 2 have primers for starting. The low and high speed windings are just that one works at low speed and one works at high speed. You need the factory Specific manual to properly work on that motor.

Did some measurements in the driveway today. 3 coil voltages are in spec. 1 coil is getting replaced as I could not get a consistent voltage across it at all. With the + wire to it was undone, I got a nice consistent voltage from the switch box. With the engine warm the high speed stator resistance was in spec. The low speed was 10,000 ohms and change. Will test it again in the AM once cold before ordering a new one.

Thx for the input.

Low speed stator was 8400 ohms cold. Spec max was 7600 so it is not that off when cold but sure is when hot. Manual says it will go up slightly when hot but is it safe to assume that a 32% increase is far too much?

Put in a new coil. The ones in there certainly look original! Voltage across all 4 is now within spec. It even sounded smoother with a new coil in it. I did not test to see if the cyl was firing as I just assumed it was not. My order for a coil included the flywheel puller so took that off and ordered a new CDI stator to be here in time for a Sat AM test run. Fingers crossed.

Took it out for a spin Sat AM with the new stator on from CDI and ran great for about 30 mins. We just did some laps around the lake at which point the same scenario happened. Could not get above 2500 rpms on it thereafter. May put it up forsale and see what happens if someone needs parts or rebuilt it over the winter.

Tomorrow will go buy a new Honda 90.

where are you located