1992 Mariner 60 hp bogs only when cold

I have a 60 hp mariner on my dad's old Bass Tracker aluminum fishing boat. This is a strange thing that happens, so let me try to explain.
When starting, motor cranks fine, no issue.
Idle, motor is fine, no issue.
Half throttle, motor runs fine...
Give it full throttle, it bogs down, and will run on 2 of 3 cylinders. No fire to middle cylinder. I know it is not a fuel issue, as I have sprayed fuel into all 3 carbs to investigate. Top and bottom nearly flood out. Middle does nothing.
HERE is where it gets strange...
Let the boat get warm, say 15-20 minutes run time, and you can hear the third cylinder come back, slowly. Firing on occasion, then better say 5 more minutes, then full bore in about 5 more minutes.
What the....

 

Get a workshop manual and follow troubleshooting procedures, would be a good approach to fixing it. Maybe a sticking needle valve, but systematically following the routine layed out in the manual is the best way.

 

Sparkplug wires.

 

Or the coil for that cylinder

 

All easily checked out with a multimeter and a manual.

 

Coils can test good but not function correctly when the motor is running.

Most electronics tend to fail as the motor warms up, but Mercs can at times not work when cold, then as the motor warms up it starts firing on all cylinders, so it could be the switch box.

Check for spark when the motor is acting up to see if it's firing on the middle cylinder.

 

Odds are if the component fails the troubleshooting test, it is stuffed. Beats guessing. Of course spark plug(s) could be to blame, and that is a cheap substitution to try.

 

A weak spark can ignite fuel mixtures at low loads.

At higher loads it is much harder for the spark to jump the spark plug gap and you have misfires and stalls.

 

Thanks for the suggestions. I should have said- changed the coil pack and plug wires. Still not firing, so I switched coil and wire with another cylinder- worked fine there. Is there an ignition module on this engine? I see a small rectangular box above the coils with wires going to each coil.

 

Switch box! That is probably it. All coils are good/new, new plugs, but this rectangular box has always been suspect to me. So it's a switch box you say. Wow! Thanks. WIll order that today.