If not "over richened" it should not kill fuel mileage. Like I said, if the car is running good now, leave it alone. The problem is knowing if your car is running as Mercedes intended. I had driven a dozen different 1987 300D's and TD's, and noticed a wide power variance between cars (at low, mid, and high RPM's). It wasn't until I bought my second one that I realized just how good the off-idle response is SUPPOSED to be. I have not touched the ALDA on that car and I don't intend to - it probably would not improve power at all. I am going to experiment on my first one though, since it is not as snappy and I want it to run as good as the second one does!
Something I forgot on my last post. I stated that the ADLA was not supposed to be adjusted - this is true. The enrichment is SUPPOSED to be adjusted internally on the pump, by a Bosch shop on a calibration bench. The ALDA controls the position of a rod internally, and the adjustment should be inside the pump on the internal linkage, not externally at the ALDA. But since it's major work to R&R the pump, plus $150-$250 to have a shop calibrate it, it's much easier to dink with the ALDA and hope it works!
Finally - the 240D and other normally-aspirated diesels only have an ALDA-type unit to adjust for altitude (if they even have an ALDA). Turbo diesels need it because of the turbo. It also compensates for altitude on the turbo motors, as explained in the OM617.95x engine manual (section 7, I think.) If the 240D mileage plummeted that low without a power loss (or smoke, etc), I'd suspect either the ALDA was damaged when adjusted (internal bladders can rupture) or something else happened at about the same time, i.e. fuel quality in your area changed, etc... 29 to 21 from an ALDA adjustment sounds pretty extreme!
