2000 Optimax 150s both overheating after spark plugs change. Need help

[JPTL]

Original OptiMax,150s on my Whaler Conquest. 415 hrs. Ran fine last season.
This season I decided to change the plugs. I went for factory recommended NGK PZFR5F-11. It had IZFR5G plugs in it. When I took them out, they all had a pretty heavy coating of black soot on the electrodes.
I figured a plug change would make these engines run better, but at this point it hasn’t. They’re both throwing the overheat/throttle down alarm (Smartcraft). It seems to come on at about 172° at about 1750 RPMs. Both engines are throwing the alarm, which is a real head scratcher for me. I’ve researched the correct plugs for these engines and I come up with both the new ones that I installed and the ones that were already in it, also getting mixed details about them being interchangeable or a plug change in the last 20 years.
Where should I start?

 

[dingbat]

I

Original OptiMax,150s on my Whaler Conquest. 415 hrs. Ran fine last season.
This season I decided to change the plugs. I went for factory recommended NGK PZFR5F-11. It had IZFR5G plugs in it. When I took them out, they all had a pretty heavy coating of black soot on the electrodes.
I figured a plug change would make these engines run better, but at this point it hasn’t. They’re both throwing the overheat/throttle down alarm (Smartcraft). It seems to come on at about 172° at about 1750 RPMs. Both engines are throwing the alarm, which is a real head scratcher for me. I’ve researched the correct plugs for these engines and I come up with both the new ones that I installed and the ones that were already in it, also getting mixed details about them being interchangeable or a plug change in the last 20 years.
Where should I start?

One plug is platinum, the other is iridium. MGK says they are interchangeable with the only difference being the platinum is gapped to 0.044 the iridium is gapped to 0.032.

Fouling typically is not a plug problem on an injected motor. One plug would be an injector. Fouling on all holes, on both motors is more than likely a fuel problem.

Replace fuel filters and run off a separate fuel supply to see if that clears up the issue. If your not running 10 micron fuel filters, you may need to have the injectors cleaned as well

 

[Chris1956]

Do the spark plugs need to be indexed?

 

[JasonDom]

I agree with the OP, stop chasing the plugs. Both numbers are correct for that engine. The timing of your plug change could be a coincidence.

Watch water pressure when the alarm hits - low PSI possibly? Check the impellers or consider the complete water pump rebuild kits and new thermostats, two per engine (stainless 143 degree ones).

 

[JPTL]

First of all, Thanks, a boatload for taking the time to reply. Good advice that I will definitely take next season when it comes to maintenance on the thermostats and impellers. But that can be done at the beginning of next season, fortunately.
I found and fixed the issue. It was solely related to the spark plug change - not the spark plugs themselves, but 100% the monkey behind the wrench who changed the spark plugs.
I decided to use a flex extension to remove and install those tough ones at the bottom on the left side. Turns out that given the angle that I was ratcheting with the flex extension I had no idea how tight I was seating the new plugs.
Going back in to check my work, it turns out that I cracked the porcelain insulator on both of the bottom left plugs, so they were most definitely arcing against the block. I didn’t mention the rough idle, because I thought it was still coming out of winterization and shaking out the oil in the cylinders.
Anyway, I got new plugs in both engines and took extreme care on that bottom left plug this time. Now the engines are running great and clocking 135° at 1500 RPM, lower at cruising speed, which seems pretty normal to me.
Lessons learned:
Changing plugs with the engine, fully tilted up, boat in the water isn’t as daunting as it seems. It’s very doable.
Don’t attempt critical maintenance when you’re in a hurry, the light isn’t the greatest and you have a beer in your hand.
Don’t use a flex extension when you’re replacing that lower left plug. At that weird angle you’re almost certainly going to crack or break that insulator…and you may not even know that you did it until things get worse.
.