Thanks for the info.
First, looks like you have the correct distributor! It is a vacuum
retard type. That is why the idle drops when you connect the vacuum.
The chalk mark that someone else made is the correct setting mark for at idle
with vacuum. It should be set to 4 degrees after TDC, and it looks like you have 9 ATDC. Advancing the 5 degrees or so will speed your engine up some and give it more pep.
You may not understand using the knob on your gun. you should turn the knob until the timing mark lines up with 0 degrees on the crank and then what the knob says is what your timing is set to. Your second picture where the crank mark is on 10 ATDC and you say the gun is on 14, that is meaningless. you should keep turning the knob until the crank mark is on "0".
To set your car,
-1. set your fuel mixture to smoothest idle as discussed in other thread.
1.
connect the vacuum line,
2. put your knob on 4 ATDC point and shoot, while turning the distributor until the mark you see is on "0". (or just set the knob on "0" and turn disti until the mark is on the chalk one 4 ATDC.
3. disconnect the vacuum line and use the 3 test values of 1500, 3000 and 4500 rpms and see what your timing is for each. These 3 test values will show if your mechanical advance is working correctly.
1500 rpm = 13-17 deg
3000 rpm = 31-35 deg
4500 rpm = 37-41 deg
If you are not able to get these numbers that means your mechanical advance is gummed up. This happens on old disti, the springs break or stuff just gets stuck.
Let's set the timing correctly and measure the test values and then see what we need to do next.
(now that all being said I found this post about a 1973 280 with a non stock carb:
1974 Mercedes 280 114 / 110 ignition timing - PeachParts Mercedes ShopForum
"The suggested timing setting is 4 degrees AFTER top dead center, vacuum line connected, idle speed 950 rpms, WITH vacuum being supplied at idle.
I set my timing ('73 280, automatic) with the diaphragm DISCONNECTED at 10 degrees BEFORE TDC. I usually plug the vacuum line and run it with no vacuum retard connected. This is because I converted from the Solex 4 BBL to an Autolite 2 BBL and I don't have all those Mickey Mouse vacuum solenoids hooked up. I hook up the diphragm every year at emissions testing but other than then I run on straight manual advance timing.
You can advance or retard the initial timing to meet your needs (for more "pep" advance it, for better mileage slightly "retard" it)"
The official MB settings may not exactly work with the carbs you have, so first try them, then maybe try to alternative mentioned above.