Rob, welcome to the forum.
First off let me say that I'm by no means a boat engine mechanic, but your story sounds just like me last summer.
I have a 2005 Merc 15 hp 4-stroke on my boat. The summer before last, it never saw the water. So, the motor sat for almost a full year without any maintenance. Last summer when it was getting time to get the boat ready for the water, the motor would not start. It would not fire at all. After a couple of days of fussing and cussing I decided to carry it to the pros. During the time I was messing with it, I noticed what looked like oil dripping from around the front of the prop. So, when I took it in to be worked on, I told them about the oil drip.
When I went to pick up the boat from being repaired, I talked to the person who actually did the work on the motor. Basically, here is what he told me:
The reason the engine would not fire was because the carbs were gummed up. He used an ultrasonic machine to get the gunk and gum out of the carbs, and replaced seals/gaskets. The oil leak was nothing. He took the prop off and checked the seal behind it, and found no leak. Upon closer inspection, what I thought was oil wasn't actually oil at all. It was a mixture of gas/oil/dirt/water that would run down the front of the motor shaft (transom side). It would collect on top of the prop assembly, run around the front of the prop and drip off of the bottom front of the prop.
He highly recommended disconnecting the gas line after each use and let the motor run itself out of fuel. Always use fresh gasoline, and use only non-ethanol gas. He stated that the ethanol gas is creating big problems with the gaskets and seals on boat motors.
I also bought a new tank and fuel line just for good measure.
After the trip to the mech, the engine ran great. Always started on 1st pull of the rope.
Fast forward to this season: Boat ran great the first couple of times out. Then one day it got to where it would not idle. At idle, it would spit and sputter and die. I just happened to notice a strong smell of gas around the rear of the boat and happened to look down and see that my 1 yr old gas line was leaking badly at the base of the primer bulb. I went to Wallyworld and picked up a new primer bulb, replaced the old bulb and it is back to running normal now. I really think the ethanol had eaten the rubber bulb up allowing it to draw more air than fuel at idle. So, check your fuel line and especially your rubber primer bulb for any pin holes and deterioration.
Hopefully your troubles turn out to be as small as mine and you'll get it up and running in no time.
