Lost #5 cylinder Mercury XR6 (carb) PIC HEAVY

[ProXR6]

S/N:0G825341
1999 Mercury XR6 (carb)
Oil Injection Deleted/Premix 50:1

This past Sunday evening i was heading back to the ramp and on the last stretch i did a short WOT pull as i do at the end of most fishing trips. Just as i was approaching 5400rpms i experienced a sudden loss of power paired with a terrible rattle coming from engine. I knew right away what happened. i limped to the ramp, winched my boat onto the trailer, and pulled the plugs when i got home. Just as i had feared, the plug from the #5 cylinder had metal shavings on the electrode. Oh, no. Monday after work i went and pulled the head off the afflicted bank of cylinders to have a better idea of the damage, and i've attached pictures of what i found.

Has anyone seen a damaged piston of this nature? Im not sure what could have caused this or what i should do from here. At the time of power loss i had over 15psi water pressure and the head gasket didn't appear to be blown. Am i looking at the result of pre-ignition? Im left at a fork in the road between rebuilding or repowering. This motor has been bullet proof up until now. I apologize for all the questions, but im just left here scratching my head.

[leonreno]

I’m definitely no expert, but you have more carbon build up on that piston than the rest, not sure why but it may have caused a stuck ring maybe. I have a 2002 Carbed 200 hp and it blew a cylinder due to a dirty jet in the carb.

[ProXR6]

I’m definitely no expert, but you have more carbon build up on that piston than the rest, not sure why but it may have caused a stuck ring maybe. I have a 2002 Carbed 200 hp and it blew a cylinder due to a dirty jet in the carb.

Well my next course of action is to dig into the reed cages to make sure none are missing, ill certainly check my carbs while im there. Ive fished from this boat EVERY single weekend for the past 2 years with the exception of a couple ill timed hurricanes and have used non ethanol fuel religiously. This motor has been rock solid every single time. i hate that this happened. I also changed all the sparkplugs this past thursday. Not sure if its a coincidence or not.

[EuropeanAM]

Don't think it was carbon OR lean. I'd be checking the locating pins on the piston, lock-rings for the wrist-pin, reeds/screws, and verifying that the thermostat on that cylinder bank is functioning properly (pending appearance of the piston once removed).

Definitely appears that it was an abrupt and short-lived failure.

[ProXR6]

Don't think it was carbon OR lean. I'd be checking the locating pins on the piston, lock-rings for the wrist-pin, reeds/screws, and verifying that the thermostat on that cylinder bank is functioning properly (pending appearance of the piston once removed).

Definitely appears that it was an abrupt and short-lived failure.

Was hoping you would chime in. I will add those to the list. Thank you for your input.

[ProXR6]

Don't think it was carbon OR lean. I'd be checking the locating pins on the piston, lock-rings for the wrist-pin, reeds/screws, and verifying that the thermostat on that cylinder bank is functioning properly (pending appearance of the piston once removed).

Definitely appears that it was an abrupt and short-lived failure.

Well i tested the thermostat on the offending cylinder bank and found that the thermostat wouldn't open until 155* and became fully open at around 165*. This is certainly hotter than proper 140-145* as suggested by the service manual, but is it hot enough to cause a failure of this nature? Im not sure. Ill be tearing deeper into the motor this weekend and i'll post what i find.

Assuming the rest of the powerhead, connecting rods, and crankshaft are in good condition, i'll more than likely be rebuilding this motor. I'll dedicate a thread to it and will be open to any advice and criticism. Between having the complete service manual, this forum, i'm confident in attempting the rebuild myself, and it would make a cool project.

[EuropeanAM]

Certainly shouldn't be that way, but can't say that it would have caused this failure unless run at low RPM's for an extended time.

[ProXR6]

Well i did some more digging this weekend, and what i found was heartbreaking. As i was pulling the reed block off the front of the case, marble sized chunks of metal started falling out. Not good. Then as i looked a bit closer, i noticed a quarter sized hole in the front of the case. So i guess rebuilding is out of the question now.

Does anybody have any recommendations on a reputable source for remanufactured powerheads?

[EuropeanAM]

There are still a few Mercury Reman powerheads available as of today's date, under Part# 823434R97.