On Tue, 1 Nov 2016, "Jack G." wrote:

Try taping over the overflow holes in the sink. It has worked for
me. A last resort would pouring a gallon of bleach down the drain
and let it sit all night and then pour boiling water down the drain.

Would tape really hold? The next problem is the sink is very small. I
regular toilet plunger is too big. Possibly something like:
http://www.harborfreight.com/power-plunger-99644.html

Then a correction. It is an S trap, not a P trap. If I remove the middle
piece, I have an open end facing downwards. But I don't know how to push
air into it.

Don. www.donwiss.com (e-mail link at home page bottom).