The mixture screw is between the mass air flow sensor plate and the "tower" where the injection lines attach. The screw you are messing around with is NOT the mixture adjustment. There no way this screw can leak fuel, there is no fuel anywhere around it.
The mixture screw will also have an anti-tamper device of some sort -- on the M103 it is a metal cover pressed into the top of the "stem"' above the screw.
The fitting the 3mm Allen goes into is a spring loaded key, at least on the M103, so you must press is down to engage in the actual adjuster. I assume the one on the M119 is the same.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!