Cold start valve for 323i

[Dan323i]

Hi, had an occasional problem with starting my '78 323i for a while now. The local BMW specialist reckons it's the cold start valve. A little squirt of WD40 down the end of it does the trick, but obviously that's not an ideal solution. Does anyone know where I can get the part, or have a part number to help me track one down? Once I find one is it an easy job to swap them over (for a mechanical doofus such as myself), or will I be opening a can of worms?