Smoke piston after rebuild

[jimmyebay]

So I rebuilt this motor over winter. Rebuilt carbs also. Set base ignition timing. Set high speed timing. Was out on river running it. Prob got 10 hrs on rebuild running 25/1 mix fuel. Got into it about 3/4 throttle for about 2 minutes. Motor rattle some and stalled. Wouldn't crank. After sitting for bit, started. Ran decent. Got home and pulled side transfer plates off. Found all piston perfect but number 5 (bottom right) scored bad. When rebuilt carbs, removed welch plugs to make sure clean and installed new ones. Jets are .002 over stock for the .040 overbore. When took carb apart last night, was careful so could inspect for plugged jets. Found none. Did notice when dumped bowl on metal top bench, had about 4 dime size water spots floating. (Was raining bad earlier that day). Possible that little bit of water kept my fuel/oil away on just the one cylinder and not the other that shares carb. Anything else I can check to prevent this again?! Thanks.

 

[jimmyebay]

Def seems like it was a lack of fuel/lean condition which means no oil. Checked piston clearance after machine shop and all the same so figure if was to tight, would of effect rest of them since all the same. No over temp alarm either and that works and sensors test good. Thanks

 

[gm280]

Unless you have individual carb for each cylinder, I can't see one cylinder running lean with all the others running perfect. If that IS the results, you have something else going on. Like maybe the water not circulating around that cylinder properly or something. What type engine, HP, model, year, size and such so we have a better idea what you're dealing with... JMHO!

 

[jimmyebay]

Huh. Must missed most important thing. It's a 1987 johnson 175 hp. Model is e175tlcua. Has new deflectors and still in place correctly. Each carb run 2 cylinders. Each carb has 2 jetting systems. Thanks.

 

[racerone]

Each carburetor bore feeds one cylinder,

 

[jimmyebay]

Yes. Each carb bore feeds a cylinder. Do you think having some water in there could siezed a cylinder? I know water is dense and heavier than fuel. So if engine couldn't lift the dense water, it would act like a plugged main jet... Agree? Still don't get why didn't effect other side but if boat was leaning it would sit on the one side

 

[Bosunsmate]

what did you seal the crankcase halves with?

 

[emdsapmgr]

You can ingest water into the carb bowls two ways. Through the fuel tank and from the oil tank. Check (take a sample) of the bottom liquids of both tanks to see if there is water contamination. Your engine stalling after 2 minutes running- could be the result of a high rpm overheat lockup. Piston got so hot in the cyl that it expanded and just froze inside the cyl liner. When that happens the engine won't crank over till it cools. Then it will run again until it overheats again, or until there is a catastrophic failure of a piston/ringset. The internal overheat occurs without warning. If the cooling system is functioning normally, you may never hear a hot horn. This can be due to misplaced diverters beside each cyl liner, or could be due to a water restriction inside a cyl head. (did you had the cyl head covers off during rebuild?) You can also get a high rpm lockup due to a hydraulic lock. If you have a water leak in the exhaust chest, that can be severe enough that it water will enger the cyl and will lock up the cyl. Till the pressure bleeds off, it won't restart. You can bed rods this way. Was #5 the cyl that failed by any chance?

 

[jimmyebay]

You can ingest water into the carb bowls two ways. Through the fuel tank and from the oil tank. Check (take a sample) of the bottom liquids of both tanks to see if there is water contamination. Your engine stalling after 2 minutes running- could be the result of a high rpm overheat lockup. Piston got so hot in the cyl that it expanded and just froze inside the cyl liner. When that happens the engine won't crank over till it cools. Then it will run again until it overheats again, or until there is a catastrophic failure of a piston/ringset. The internal overheat occurs without warning. If the cooling system is functioning normally, you may never hear a hot horn. This can be due to misplaced diverters beside each cyl liner, or could be due to a water restriction inside a cyl head. (did you had the cyl head covers off during rebuild?) You can also get a high rpm lockup due to a hydraulic lock. If you have a water leak in the exhaust chest, that can be severe enough that it water will enger the cyl and will lock up the cyl. Till the pressure bleeds off, it won't restart. You can bed rods this way. Was #5 the cyl that failed by any chance?

Yes it was cylinder number 5. Why ask? Something common? Don't believe was a hydrolic lock up. Rod would of bent. Plus piston was extremely scored and aluminum transfer to cylinder wall from piston. All Diverters are in place and new. I did removed the head covers when overhauling. New thermostats and pop it valves. I'll prob pull cover off head again to double check things. Curious why ask if cylinder number 5. Let me know. Thanks.

 

[emdsapmgr]

So the failed piston was the overbore one. It's possible that when the overbore was done that it was not bore sized to accommodate the diameter of the overbore piston. And perhaps the specific expansion rate of that new piston. If the clearance is to little, as the piston heats up, it will expand and can scuff the cyl liner due to heat. Almost regardless of fuel/lubrication. If you had a "tight" piston and add the possibility that there is water in the fuel (lack of full/proper lubrication ratio in the fuel) it could scuff that liner and piston. Perhaps your machinist went over the overbore procedure with you during the rebuild?