Motorcycle Repair: honda metropolitan 50cc 4 stroke, liquid oil, black as night

Question
Falcon-

Ok girlfriend comes to me to fix her scooter that is running like crap.  

As I'm pushing it up the ramp liquid comes out of the air cleaner.  IT looks like oil, but I'm not positive.

When I take off the air box cover a considerable amount of this "stuff" comes out.  I figure she must have sprayed water into the box when she was washing it or something.  So I replace the spark plug (black as night) and the air filter (soaked).  Runs great for 20 minutes or so.  

Today she comes back and it's running awful again.  Especially at full throttle. (dies)  Black liquid is also dripping out of the air box.

When I took it apart today I could tell the black liquid/oil is coming from the carb.  

Any ideas?
-Ryan  

Answer
Hi Ryan.
 What you're describing is clearly not from water, but it does sound like there is oil getting into the air box.

 Perhaps she's overfilling the engine oil.  That would definitely make it suck oil into the airbox.

 Are you sure that the bike is a 4-stroke and not a 2-stroke?  I don't recall ever hearing about a 4-stroke 50cc scooter, so that's why I ask.

 Perhaps she's putting oil into the fuel, thinking it's a 2-stroke (if it is a 4-stroke) or putting too much oil in the fuel (if it is a 2-stroke).
Is the plug just blackened with oil or is it carbon?

 You should ask her if she's mixing oil in the fuel and she should get the repair manual for the scooter.

Check those things and let me know what you find.
Good luck.
FALCON