D-jet v8 needs massive timing and proper mixture set up.
The timing is of utmost importance. Pull all the vacuum lines off the distributor and set the timing to 15 deg BTDC. Be sure it gets at least 10 degrees more timing when the engine is raised in rpm to 3000. Total advance at 3000 should be at least 30 (I think).
Once the timing is there adjust the mixture to maximum vacuum. You should strive for 17in manifold vac at idle. For best results use an exhaust gas analyzer and disconnect the idle switch (this removes the idle mixture compensation circuit. Set the mixture at the manifold pressure sensor to less than 1% CO and reattach the idle switch. Then set the mixture at the controller to 2-3% (best idle). For maximum fuel mileage use the less than 1% base figure. For best power set to 1.5% CO.
If you achieve 17in you will probably get 14-16 mpg on the road. All this presumes good manifold pressure sensor and proper fuel pressure.
Bosch Master, ASE Master, L1
33 years MB technician