I had some gas seeping out the carb bowls, so I took it to a marine shop, they fixed the leak by replacing all 6 bowls. They also said they ran a bottle of tune through it. I took the boat out to the lake and it idled great. After my number was called ( in a tournament ), I put the throttle to it and it got up on plane to about 4500 RPM then the RPMs started to drop and the motor locked up. Took back to the shop and they said it wasn't anything they did and was not sure what would cause it as they have never seen one do what it did. The number one piston is all chewed up around the edges, the rod broke, and went through the front of the motor breaking the throttle bodies and top two carbs. The pictures are hard to describe. I will gladly email pictures if they would help.
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rpm and starting to drop sounds like an overheat and then a piston seized and then the rod snapped. Or a oil failure
Im sure others will have other opinions but a tune up shouldnt cause that
An interesting failure. The piston crown shows some signs of rusty water. Do you know if those water marks were there when you pulled the head, or has the powerhead been sitting outside, uncovered? If enough water got into the cylinder, a resulting hydraulic lock at 4500 rpm's could yield those explosive results. Once you get the powerhead disassembled, you will be able to look at the crank and maybe some of the remaining rod. (Is the rod cap still bolted to the crank?) Would be interesting to see if any surfaces show signs of blue color-extreme overheating/lack of lubrication. Crank journal surfaces abraded? Any signs on other rods? The metal on the crown of the piston are likely pcs of the crank. Hard to tell if the piston was running lean, since most of it is covered by schrapnel. Once you clean that off you may be able to see if he piston was running lean. Other piston crowns look fine, so hard to say that it was a general lube problem. If the internal damage was limited to that one cylinder, best to pull the carb apart that feeds that cyl and do an autopsy on it. Pull the jets out (esp the high speed jet) and see if it's restricted with any debris. Any restriction would certainly cause a lean condition. Year of the engine?
If it was ingesting water, well there's your answer. Water rusts bearings, rusted bearings seize and bust the rod--after pieces of them go through the ports and destroy the piston. Not the first engine that did that, and certainly not the last.
IMO, The piston seized in the bore due to old age, a ring broke, a sudden overheat, or sudden lean condition, also possibly due to a prop too big, loading the motor?
The piston stopped, and the rod broke causing catastrophic damage. There are no signs of lean running, all the pistons look like textbook color combustion!
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