1985 115 hp Evinrude piston burn HELP!

Any Thoughts?

I have had this engine since new, ran great for 21 years. Then last July the lowest piston melted on me when I ran out of gas. Just before the engine died I heard a pop. Taking the engine apart the #4 piston (lowest) was melted on the intake side. I since rebuilt the engine and carbs. I "Broke" the rebuilt engine in as recommended by omc and at about 11 hours running time the samething happened. When running out of gas I heard a pop and the same piston had melted.

When rebuilding I put in the low compression head gaskets so I could run low octane fuel. I also checked the timing, replaced the thermostats, water pump, cleaned and rebuilt the carbs, verified the VRO was working, etc.

After rehoning this time so I would have a good surface to mic I noticed a small 1/4 inch crack in the sleeve starting at the head mating surface and proceding in the direction of piston movement.

Do you think this crack could cause the detonation? If some (very small amount) of water got into the cylinder? If not any thoughts why I burned the piston in the same cylinder again?

I plan to check fuel pressure when I get it back together.

Since the sleeve needs to be replaced any machine shop recommendations? prefer local to the Boston Area.

Thanks
Bill

Re: 1985 115 hp Evinrude piston burn HELP!

You say it burned the piston when running out of gas??? I have heard if you run some motors out of gas that one cylinder will lose gas first and possibly burn up the motor. I would have to assume that this is the reason that it happened. Since both times you say you were running out of gas. As to the crack in the cylinder sleeve I would associate it as being "caused" by the running out of gas, Not the "Cause" the burnt piston.

Of course I am not expert in this but I know that there are several on here that are. So if anyone feels that I am wrong (or right) feel free to correct me.

Re: 1985 115 hp Evinrude piston burn HELP!

I think you have something else causing the meltdown. Maybe lack of cooling water to that area. Check the deflectors and the heads under the water cover. I assume you replaced the Tstats and waterpump with the rebuild
I'm personally not convinced that running an engine dry will cause any harm. The oil coating on the internals just doesn't disappear in a minute or so unless its washed out by pure gas or some other solvent. Even if it did your damage would be accumulative over many cycles of running the engine dry, not a sudden failure like you had. I'm not sure I'd rebuild a 22 year old block that has had 2 failures. I'd look for another clean block to start from.

Re: 1985 115 hp Evinrude piston burn HELP!

darrklim2

Thanks for your comments, that is what I was thinking also. I was planning on putting a next size up high speed jet (if I can find one) in the #4 carb to help prevent it from happening again. Not sure if I should put it back together with the crack.

Re: 1985 115 hp Evinrude piston burn HELP!

Walker

Ya thats what I am pondering but haven't found any reasonable prices for a short block, any recommendations or sources?

When I rebuilt it last time I did check the water deflectors. I also took them out today and they looked fine.

yes I did replace the water pump, thermostats and relief valves.

I wonder if it could be the gas.


Bill

Re: 1985 115 hp Evinrude piston burn HELP!

YOu wrote "Do you think this crack could cause the detonation? If some (very small amount) of water got into the cylinder? " - - Yep - that would do it.

Re: 1985 115 hp Evinrude piston burn HELP!

the #4 piston has the least amount of back pressure in the exhaust,if the fuel mixture is lean thay cyl will die first.on the new powerhead step the high speed jets up 2 sizes and retard thetiming 2 deg to be safe.1985 engines were not set up for 10%+ ethanol in the fuel.