This week I purchase a 1974 914 2.0L. Looking over this project I noticed most of the vacuum lines were missing so I purchased a kit from AA. While installing the new vacuum lines I notice the decel valve is missing. So the question is do I need one and if not should I plug the vacuum lines that would normally go to the decel valve? If I need one, suggestions on a source for one.

Thank you for your help.

Yes, if you still have the djet and mps. If you have carbs... No.

There is a big debate on this parts need on the engine. The early cars with less emissions rules did not have them. As the emissions laws got tighter and tighter this was a add on to meet requirements. Our early car runs fine without it, Some say it protects the MPS but this would not be it's original reason for Porsche adding it to the car. If it were me and I wasn't concerned with originality, I'd plug the line and drive it.

You don’t need it, but there’s no benefit in not having one unless you have an early car and are going for 100% originality. With the decel valve the emissions are improved, the RPMs come down slowly when coming to a stop instead of almost hitting zero and stalling the car, and it may lengthen the life of the MPS diaphragm.

So I have a 73 2.0 Djet with the decel valve. When I come to a stop my RPMs come to zero and then bounce back. Is this an indicator of a bad decel valve?

Check first to see if it's hooked up correctly. The end ports should both be plumbed to the plenum and the side port should to go the air box.
If it's connected correctly, it's either bad or it needs to be adjusted to open at the right level of vacuum (~17 inHg).
More info here including how to calibrate: https://members.rennlist.com/pbanders/DJetParts.htm