i noticed the other day that there was a grey sheen of something on the water surface around the boat. the boat lift isnt in yet, so for now it just sits in the water on the dock. I pulled the motor out and let it sit for a night, next morning the sheen was gone. put the motor back in the water and now the sheen is back. is this oil? fuel? what should I be looking into? its a 25hp merc. tiller on a 14ft lund.

thanks.

It could be unburned oil on the motor.  My 25hp Evinrude some times has unburned oil on it (on the lower unit).  I wipe down my motor each time I am done using it and I have not seen the problem since I started wiping it down.

I will look into that. thanks.

I also have a 25 hp mercury engine that I notice a sheen of oil/gas around sometimes. However, if your engine is a 2 stroke like mine, then this should be normal. The oil/gas sheen that you see is due to the design of a 2 stroke engine. Unlike 4 cycle engines, 2 stroke engines lack valves.  Each time the cylinder in a 2 stroke receives a fresh charge of oil/fuel/air, some of it inevitably leaks out of the exhaust port and in to the water before it is ever compressed by the piston and then ignited by the spark plug. This results in the oil/gas sheen that you see around the engine. This is also the reason that 2 stroke engines are considered less "environmentally friendly," as they leak some unburned fuel and oil into the water. Hopefully this helps you out a little bit. By no means am I an expert on this stuff, I just have a basic understanding.

"HowStuffWorks" has a great write-up with good animations about the workings of a 2 stroke engine just in case you are interested. Check out the link below.

http://science.howstuffworks.com/two-stroke.htm

Take care and I hope this helps!

Jonathan  

You can check your gear oil too, check your fill and drain plugs