The other side of the V in the electrode is shining to the camera. I thought you had something. Then I blew up the photo.

Bill, what heads are on this 600hp 515ci? Are you sure the heads don't call for a .750 reach plug, and not a .460 reach like you have there? If they are stock merc iron heads, than disregard. But most aftermarket heads call for a .750 reach plug.

I have not, but worth a try

They are standard iron heads. 088 I believe.

Plugs look fine. I think you have something else going on. Odd that it's both engines but don't rule out another issue

Go back to mr43t. Personally I've never had good luck with champion plugs for the very same reason.

Good ref. Btw ck the ground straps on all your plugs. There difficult to see in pics. I think I would have to agree with others here also. Throw a HEI in for a quick test if doubts. Quick simple test.

Plugs look lean. Sounds like you have water in fuel. Dump water separators in a clear jug. Let sample sit for awhile and look for signs of water. Try running motors off an auxiliary tank with fresh fuel. 10 gallon outboard tanks work well.

Are the R43T the same heat range as the 8's in NGK? I'll have to look it up.

I don't have another distributor laying around, though I plan to thoroughly inspect all aspects of my current distributors, power and ground feeds and all connections.

The engines are lean at the cruising speed and will be fattened up before any further testing. I will check the separators. Obviously water in fuel is bad, but wouldn't it show up immediately or at least after the next run even with new plugs? I have not popping or back firing that I would expect with water.

Again, thanks to you guys and everyone for the input!

I run the MR43T in my blown 454s at 5psi with no problems. Try champion in one motor and ac in the other motor.

I know who to call when I need my plugs changed...............Bill I may have a merc dist. if you want to try something like that.