Have you connected a vacuum gauge? It's a cheap, and very useful tool. You might consider splashing out a few more dollars for a mighty-vac, which will also test vacuum operated devices.
It's a great old-school way to test all kinds of engine issues.
Here's a great site that give some clear examples of how to interpret the readings:
How to Use and Interpret a Vacuum Gauge
BTW, have you examined the rubber seals around the injectors? I remember working on a 78 450SL that had a poor idle and found those seals to be the culprit.