What would cause a piston to melt

[nophun]

besides a severe lean-out?

Motor is a '85 Johnson 150GT, upper starboard piston and head show detonation damage (cylinder wall undamaged!!). I'm thinking a clogged jet and will completely go through the carbs of course, but is there a possible other source?

I am well versed at automotive engine repair and have seen this sort of thing on turbocharged engines; 99% it's lean out / detonation damage, just checking to see if there's something 2 stroke specific I may have overlooked.

 

[Monte1961]

Re: What would cause a piston to melt

what fuel ratio? Should be 50:1. Timing to far advanced. And of course running to lean.

 

[emdsapmgr]

Re: What would cause a piston to melt

Likely the top piston ring broke and disintegrated in the cyl, causing dents in the piston and head. You may find bits of rings inside the lower unit exhaust cavity, behind the prop. A lean condition in the carb is a good place to start. Pull all the jets out and visually check them. You could also have a hot condition at that one cyl which will not set off a horn: 1. You could have coked rings from excess carbon buildup. Pull the side intake (bypass) cover off and look at the rings, piston skirt. Carbon buildup will prevent the incoming fuel/oil from lubricating the ringsets while it runs and it will run hot. 2. Also, you could have a rubber water deflector out of position in the powerhead which will cause that one cyl to run hot-but may not set off a horn.

 

[nophun]

Re: What would cause a piston to melt

I guess I should have qualified this more; I horse-traded into this boat/motor combo and the PO had already removed the powerhead and partially disassembled it. It's up to me to get it back together and running and I don't want the same to happen. The VRO has been bypassed and the boat's tank is full of mixed gas, but who knows if ratio is right (will discard and start fresh).

The motor was rebuilt w/.030 over somewhere along the way and what I can see of the pistons there is no noticable wear,there is carbon staining but it doesn't look built up at all; I'd like to try to get away with only working on this one cylinder rather than a full disassembly. There are some small tubes in between the cylinder in the water jacket; are these the water deflectors? I can pull these right out with my fingers and figured thay should be harder to remove than that, no? I won't discount the broken ring until I get the piston out, but the damage doesn't "look" mechanical at all, just melted.

An additional question; what should I use as assembly lube when I put things back together? I'm thinking motor oil is NOT the right thing, 2-stoke oil, gas-oil mix?

 

[nwcove]

Re: What would cause a piston to melt

have never seen a melted piston in an outboard, but have seen many in snowmobiles, the head damage is usually caused by the molten aluminin from the piston. its normally a lean situation, and if one piston looks perfect, and another is destroyed, a crankshaft seal could be letting air in, and leaning out on one cyl. in my experience with other two strokes, its almost always the magneto/flywheel end of the crankshaft seal that fails.....wish i knew why!

 

[emdsapmgr]

Re: What would cause a piston to melt

Those small rubber hoses in the powerhead are the water deflectors. Easy to replace them. The can get out of position or they can accumlate debris, disrupting water flow to one cyl. When this happens, one cyl can get hot and you may not get an oveheat warning horn. A clogged jet in the carb will lean a cylinder out. Again, probably no warning horn. Pull that carb apart and visually check all three jets that feed the bad cylinder. Make sure the passages in the carbs don't have any dried fuel or debris (like old fuel hose pcs) clogging the passages. Spray some gumout through a passage on one carb, then compare the outflow to the same passage on another carb. They all should output the same spray volume. Go on to the next passage, etc.