1991 FXSTS. 12,000 miles. Not started since last summer. Just changed the oil & filter and started it up to let it run for a bit. Everything seemed fine. No leaks anywhere. When I went to look at another bike, I saw a puddle of oil under the bike and went over to investigate. It appeared to be leaking from the carburetor cover so I took it off and removed the air filter. I noticed a small opening on the left side of the housing from which a metal line is attached, and runs down to the bottom of the engine. Sorry, I don't know the correct names of these things but hope you know what this is. If it matters, I had the carburetor rebuilt 2 summers ago. Also, the temperature outside when I started it today was about 50 degrees. Can you tell me what this is??

I can post a pic if that's more helpful.

 

Your oil drained out of the oil tank through the check valve in your oil pump into the bottom of the motor. When you started it up the oil blew through the breather and into your air filter and the onto the ground. This can easily be fixed by lapping the check ball seat. It is a common thing and will be easy to fix

 

Because the bike sat for a while,oil went by the check ball and into the case. They call it sumping. Then you started the bike and the oil was pushed out the crank vent out the breather. Happens to a lot of people,the ball may not be sealing properly.
Now your oil level may be too much and may need to be changed again if the old oil was really dirty.

 

Thank you Glidein Wide & Gerald Martin!!!!

 

You may not have a problem at all...it is virtually impossible to get one of these bikes not to "sump" at all...if it can sit for a couple of months without blowing oil out the breather, I wouldn't touch the checkball.

 

I thought 91 was a bottom breather?

 

How about a pic of the bike!

 

They are and that's the part of the problem.
The rest of the problem is OP has his breather routed to the carb instead of atmosphere.

 

I almost don't want to say anything but.....my bike has only misted oil to the breather one time and it was because I over filled the oil by mistake. It did blow oil out the vent when it had an open route with a small filter near the rear wheel.That's where it was when purchased. But I fixed that problem right away by mounting it to the back of my K&N filter. The air filter is always clean when I wash it,and it has never sumped sitting through a winter before oddly enough. Top breathers and umbrella valves...problem.
And all was well before the rebuild also,it can't be luck because I just ain't that special.

 

Might have been too much oil in the motor. 3/4 full is my max fill cold