Older Chris craft with newer MerCruiser 502s that according to the previous owner are now 540s with a healthy cam. Had AC, Delco, R 44T spark plugs. They were fouling I had to constantly Rev-it to clean it out. Doing some research, seems like a lot of people were happy with the AC Delco platinum rapidfire #1s. Put those in and runs way better, but now it randomly loads up on fuel and dies at idle.

What’s everyone’s favorite plug for this set up?

i use ur5 NGK in our 540 and it rocks no plug fouling they fire all the way to 7 grand i never had any luck with AC

What do the plugs look like after a run. Are they burning clean? Based on the pic and your post it appears you have the problem idling around.

What carbs are you running? I would be checking carbs to start and make sure they are set up right.

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Stock merCruiser MPI (no carbs)

Have you considered an ignition system to get multi spark at idle? MSD,Accell, or whatever you like. We’re ECMs retuned?

Since it's MPI, what's engine vacuum especially at idle and lower rpm. do you know cam spec's?

just tossing idea out since engine has BARO/manifold sensor?

If the previous owner went up to 540ci engine with "healthy" cam, there had to be some type of tune. Seems like the tune is off to me.

I forgot to include in my post, it does have an MSD ignition. I have to look to see what it is.

have to agree with those above. you dont have a plug issue, you have a tuning issue. you are going to have to find out what was done. if they are 540 with bigger cams what tuning to the ecm was done? if none then the ecm is confused by the info it is getting

Computer tuning 1st.
Paper note: Most marine engines using ac plugs use 43 heat range pkugs. These are 1 step colder than your 44’s.