What year is the Mercury? How does the motor run in it's present condition?
From my experience, Two-Stroke outboards are about the most inefficient engine ever designed (something like 10% of the total fuel you use to power it is actually being turned into energy). A slow leak of fuel around the manifold is usually pretty normal on an older two-stroke outboard. Not to school the wise, but the reed valve that opens and closes as your engine runs and pulls fuel into the carb, it doesn't shut perfectly, so you have unburned fuel that "seeps" out.
There's a possibility that you have a clog somewhere in the fuel delivery or maybe a dirty carb, but you would likely have a poor idling/running engine.
I'd say that if the engine runs well, I might get a second opinion, but I personally wouldn't worry about it.
