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I wanted to clean out the carburetter of my 8 hp outboard. The mechanic advised strong Seafoam mix, so I used 1 gallon of gas with 16 ounces of Seafoam. But I forgot to put any two cycle or oil in the gas.

The motor started well, and idled smoother and smoother as time went on for about 10 minutes. I slowly revved it up & down. Shortly after that it just died.

After about two minutes, it started again and ran for about three minutes and then died. Only 1/16 of an inch of gas left in the tank, so I figured it was out of fuel. (But that's OK because I just wanted to get that seafoam in the carburetter). I let it sit.

Anyway, after it died, I noticed smoke coming from the top of the motor, and it was very hot. Do you think any damage was done?

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    Maybe heat damage from lack of lubrication. If it still turns over when you pull the starter cord then it's not seized (yet). Let it cool, flush everything out, put in proper fuel/oil mixture, and see if it starts and runs. – Jimmy Fix-it Sep 8 '17 at 16:30
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    There's no way to say from here. Either it died because it ran out of fuel or you've scored the cylinders and lost compression. You'll find out when you try using it again. I don't know whether Seafoam is a good oil substitute, but I somewhat doubt it. – isherwood Sep 8 '17 at 17:02
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    Seafoam has cleaned many a crudded carb for me. It's good stuff for its intended purpose. – isherwood Sep 8 '17 at 18:08
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    @noybman (and whoever upvoted that comment) It's a 2-stroke. There is no oil in the anywhere unless it's in the fuel. There's nothing to drain, there is no dipstick. Perhaps you are not familiar with the concept of 2 and 4 stroke gasoline engines. I can assure you that anyone who is even cursorily familiar with them knows perfectly well that not putting oil in the fuel means there is no oil in a 2-stroke motor. – Ecnerwal Sep 9 '17 at 1:06
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    We have managed to answer many questions about lawnmowers, chainsaws, snowblowers, generators, string trimmers and similar home-related engine powered equipment in the past. While it's reasonable to consider "a boat motor" off-topic, "general knowledge of small gasoline motors" is very much a homeowner skill for many homeowners, and there's really nothing boat-specific about the question. – Ecnerwal Sep 9 '17 at 1:47
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Yes, it's very likely damaged. Running with no lubrication will do that. Look in the mirror and give the person who did that to you a stern talking to.

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Because the concentration of oil in a 2-stroke is so low - 1:40 or even 1:50, depending on the engine, it's easy to think that it isn't necessary.

Not so; it's vital.

Running a 2-stroke without lubrication is not good. If your engine will still turn over, it isn't seized, and you may have dodged a bullet. Put the correct oil/fuel mix in and see what happens. Because it's a large engine (as opposed to the 26cc motors in trimmers etc), you have a (slightly) better chance of getting away with this unscathed.

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It's almost certainly damaged. The purpose of the two-stroke oil is as follows:

Lubrication: It needs this to provide a layer of oil between the cylinder walls and the piston rings, otherwise it's metal-on-metal, causing high amounts of friction.

Cleaning: Any filings from the piston rings will need to be cleared away by the oil as the piston comes to TDC from the cylinder walls, otherwise it's going to create a lot of scores in the cylinder wall.

Sealing: It actually acts as a liquid-type seal between the piston rings and cylinder. Not that it could hold back the explosive force of the air-fuel mixture being ignited, but it's purpose is still there. That's why two-stroke oil is somewhat thicker than it used to be.

Cooling: Well, it's in an engine. It's kinda vital to keep something working with EXPLOSIONS to a certain level of COOL. If not, it'll overheat.

Without lubrication, it'll cause high friction, causing heat and causing metal filings from the piston rings knocking around, and losing a seal. In a way, oil counteracts all of these.

If it'll still turn over, flush the fuel, and put in the correct mixture of petrol and oil. If it runs, it means you've got a chance of it working still. If not, you've gone and stuffed the engine. Big time. Really badly. Go do what Ecnerwal said about looking in the mirror! ;)

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