2007 Mercury 2.5 4-stroke engine leaks gas and dies

[MotorMan101]

I just got a Mercury 2.5 horsepower engine and took it out on my sailboat after test running it three times. I went to start it on the sailboat it ran for about 30 seconds then died when I got home I checked the fuel tank and it was empty. I knew I'd filled up the fuel tank before I left so I refilled the fuel tank and tried to start it no luck. I checked the kill switch pendant was in position and it was, the gas was on and the choke was out, throttle was in the correct start position. I tried about 10 tries no luck. And I put the choke in try again no luck. Pull the spark plug out found it was sort of black cleaned it put it back in still no luck. Tried sparking against the engine no spark. Well I was working on the engine in the test tank I noticed that it was sporadically leaking what I thought was water but turned out to be the fuel overflow valve. So the carburetor is getting fuel filling up and dumping it about every 20 seconds. I'm not sure if I have one issue here or several issues... Has anybody else dealt with this before... I'll break it down into three questions.
1. Why is the fuel leaking
2. Why don't I have spark when I try it against housing. 3. Why does it run for 30 seconds and then stop even though the gas tank is full with the vent open I figure it's draining the carburetor but it should be filling right up..