p4-33

HP500 Carb motors, otherwise stock, and this has happened three times in the last two years. Porcelean blows out of the plug base, and I'm trying to understand if the damage to the electrode is a result of cylinder detonation or impact with side of the boat.

Changed the plug and seems solid again.

Anyone seen this?

Thanks,
Brian

Griff

I remeber hearing about this happening more often with a certain brand of spark plug. I don't remmeber which brand though. What brand are they???

ragtop409

Cant answer any questions but I can tell ya a buddy of mine had an Autolite do the same thing in his strocker motor small block. We never figured out why? Rag's

THRILLSEEKER

looks like you are way lean

p4-33

The plug pictured is an AC Delco. I'm currently using Champions.
Doesn't seen to matter what brand. Random cylinders, too.

Earlier this spring I had a piston detonate in the #6 hole, but the plugs have only failed in #1, #2 and #4.
While the motor was apart, I saw no other evidence of detonation in these cyls.

Timing is 34 @3000.

Only happens on one motor, too. No problems with the other one.

This is kicking my ass, and I lost a race this weekend because of it.

Should I be looking at jetting? Flaky fuel pump? Ignition upgrade? what are the "hottest" plugs out there?

Thanks again,
Brian

formula31

Your not running an MSD box are you?

p4-33

No, but considering it. Why?

Right now, the carb, dist, coil are all stock HP500 stuff from Merc.

p4-33

Oh, the fuel pumps were switched to Holly Blue's by the previous owner.

formula31

Sometimes the electronic boxes will intermitantly advance the timing enough to cause problems. Ive never heard of a Merc box doing it but you might want to change it to be safe. Never had a plug do that.

Turbojack

Detonation is what is tearing the plugs up. The good part is the plug is taking the hit. Detonation can/willl destroy an engine quickly. Need to find the problem. Hotter plug is not the answer. When you had motor apart could you get a guess on the compression? I would check the jetting.