I saw a video on youtube about how to crank a merc 2.5hp.. The guy opened vent on top of gas cap, opened valve between tank and carb,, pulled choke out, turned tiller throttle to start...pulled starter rope a few times..did not crank..he might have pushed choke back open?? Then he opened throttle a little more and it cranked. I would change the spark plug, I got a bad one once.