4cyl 2-stroke WOT spudder when hot

[Gin]

Greetings!

Just seeing if anyone might have some ideas...

My old '84 4cyl 2 stroke merc starts to sputter at WOT after running for 5-10 minutes. I pull back some on the throttle and the sputter goes away and I can cruise just fine. Motor has good compression, starts fine, idles good. Primer bulb stays firm, replaced the fuel pump diaphragm, plugs, wires etc. Pretty much only is only a problem when I want to let it scream. The plugs look kind of sketchy if I'm being honest, bottom plug looks too clean. I've ran this motor with this condition for a few years now on the weekends and it never seems to get better or worse. My gut feeling is water intrusion given the delayed timing but its just a hunch. Attached a pic of the plugs, 1-4.

Appreciate any thoughts.

[Gin]

I guess my other hunch is seals and lean condition

[mercrazy]

Run it on a remote tank.
could be break in trigger wiring at wot.

[Jim Speros]

Give the plug wires a good look. If your brave touch them while engine is running on hose and see if they bite you. If they do they are bad.

[oldschoolnc]

I'm agrre w/mercrazy on try remote tank.
Pump the ball while it's occurring, see what, if anything changes.
Plugs look well used...

[Gin]

Appreciate the replies! I tried running a remote tank and it didn’t seem to help. I’ve also tried squeezing the bulb and that doesn’t seem to make much difference. The time component is what makes this hard. Thing runs great for the first couple of miles out of the launch. I have another running parts motor I can try swapping all the ignition stuff over. I wonder if the switch box could get hot and start to maybe not keep up with the higher rpm’s…idk. The plug wires appear good to me.

[skialot2]

I would try new spark plugs. If that doesn't work I would try switching the coils from your parts motor. Coils can do funky things when they get hot.

[Gin]

Tossed new plugs in and swapped coils and no changes. I’m thinking I might try the remote tank once more just to make sure I’m not crazy. I love these old mercs but man sure can be frustrating

[mercrazy]

Could be switchbox or stator

[Gin]

I replaced the switchbox and lo and behold, hasn’t missed or studderd since! Fingers crossed it stays that way. I really appreciate everyone’s help with it.