If the engine is rich enough, it will spit out unburned fuel as well as oil, espeically when you first get it started.

It's also possible that you are getting fuel in to the muffler, either during fueling (by not taking the vent line off the muffler), or when starting (most common with a side-mounted engine, but possible with some muffers on upright mouts).

As suggested above, run the engine up to full power, then slowly lean it out. The RPM should go up and the exhaust will change.

Oh, and finally, I assume the fuel is actually coming from inside the muffler? I've seen a pinhole in a fuel line squirt fuel out so that it winds up running down the outside of the muffler and into the exhaust because of the prop blast. At first glance it looked like it was coming out of the engine, but it was really all external.