I just installed a laundry sink in my basement using the pre-existing 2 inch drain pipe that was coming out of the cinderblock wall. I also put in a washing machine and stuck the drain pipe into the sink. The problem is the laundry sink drains very slowly. I'm assuming the builder connected the drain to the vent pipe but this was a long time ago so I can't ask him.

Is the lack of a vent the problem? And if so, how do you install a vent so the sink will drain well?

Thank you!

Is the lack of a vent the problem? And if so, how do you install a vent so the sink will drain well?
Several things come to mind.
1)The unused pipes could have construction debris in them causing a partial clog. Try snaking thye drain line to see if that helps. **OR**
2) If you're not vented you could install a AAV, (see image) just downstream from the trap. Good luck, Tom

Not necessarily: Typical laundry sink has no overflow. That makes it to drain slowly - especially if your laundry sink drain is 1 1/2", with strainer, and you drain your washing machine into it. That only magnifies the slow drainage...