1975 450 SL runs too rich--need help

Have 1975 450 SL with 120K miles.
Engine is mechanically sound and well tuned.
Runs great with no visable emissions.
However it has been tagged as a GROSS POLLUTER here in California because it runs to rich at higher engine speeds. (it's fine at idle and just bearly legal at 1500 RPM. The faster it runs the richer it gets.)
This engine has the D-Jet injection system that was only used for a brief time.
I have a good mechanic but he is presently stumped.
He has checked fuel pressure and any other available adjustments with no luck.
He is about to begin replacing parts in a trial and error manner and they are all expensive.

Would appreciate hearing from anyone who has sufffered and solved this problem.