That chassis uses vac retard, not vac advance.
So, you set the 30 BTDC @ 3 K FIRST and then if you want to set the idle timing , you change that timing by the adjustable rod on the dist vac. servo element. It is threaded for length changes. The point is, once you get your 30 set, you lock down the dist. and leave it there. The low end adjustments are not made by turning the dist. [ Your 3ATDC is actually in spec for that car..2-4 ATDC @ idle]
At idle , if you take the vac line off dist, the rpm should speed right up b/c it is then advanced [ no more vac retard]
If it is a stick trans a good setting is 30 @3k and zero[tdc] at idle...
Make sure the throttle plate is all the way closed at idle. A trick for that is to take the intake air filter hose off and place a flat piece of cardboard in front of the throttle intake, blocking off the intake air. There should be very litte, if any, difference in idle rpm ,when coolant temp is normal, with/without blocking intake b/c the engine gets all its intake air from the air bleed screw. If all is correct, You should be able to stall the engine with the bleed screw by turning it in ..if NOT, then you have other intake air, which can be caused by a not fully closed throttle plate OR the WRD is not shutting off intake air when coolant temp is reached to normal spec.
It is the closed throttle plate that causes the highest vac to reach the vac element at the distributor and when it does , it RETARDS the ignition timing.
This is a great system b/c as soon as one opens the throttle , the vac drops off instantly , allowing for a sprk advance ....from that point , it is the weights in the dist [ mechanical advance] that effectively control the advance curve...the trick is to set the 30@ 3k FIRST b/c that is a constant advance you want at 3K, regardless of where you decide to do any tweaking on the dist. fine tuning of the rod length.
The reason for Factory spec of ATDC @ idle was mainly early emmisions
and proper idle, specially w/auto trans. Earlier version distributors had vac advance...I like the retard system better.
To answer your timing preference question, I personally like to set idle timing to Zero [TDC] w/vac on my 250sl, which is basically the same set-up and MFI pump/engine as your 250se....and I have a manual trans, so I run 800rpm and slighly rich on the bleed screw for best results. [ meaning, you are looking for a higher than needed rpm with bleed screw, and then back the idle down with less air from bleed screw, resulting in a slighly rich idle setting...this assures you are not lean at idle and on that R18 pump, the transition to the midrange is much better with the low setting on the rich side b/c those pumps were slightly lean on the midrange [ that was corrected on 18z pump, so check your pump#] ..a vac gauge works well here. Peak the vac @ idle and then richen just a little by turning the bleed IN w/o loosing vac reading..you can hear the exhaust notation drop. That is the sweet spot. If the idle rpm is correct only when the bleed is at its highest obtainable rpm, then you ate actually too lean..you want that final tweak to bring the idle to correct rpm by slighly enrichening via the bleed screw. A 50-100 rpm drop from highest bleed setting is usually about right .
You mention access to a gas/A , so if you use that , you want the idle settings on the high side ..like around 4.5% CO.. You know how to adjust the pump thumb screw, so after that, get the bleed as the final tweak.
And any/all of those adjustments aredone AFTER ign specs are set. or you are wasting your time. I always do both ign and valve adjustment before any a/f mix settings. And those start w/linkages.................
Be aware that most of these pumps wear towards rich anyway. That can all be adjusted and I am sure you have seen that info.
As far as your pinging test, shifting into a lower gear is not a good test for ping...you want to stay in highest gear and go up a slight hill ...if there is pinging, it will appear as you attempt to add throttle while the engine has the load of BOTH high gear and hill inclination combined . That is extreame Load condition on engine.
I own a 121 chassis [ 1956/190sl] that actually has a cable running from the dist to the dash , where the driver can change the ign timing from the cabin in situ...the owners manual gives specific hill incline/top gear instructions on how to turn the control knob back one click for proper timing after pinging is detected,,,,,,,. pretty cool.....They did that back then b/c octane ratings were all over the map from one station to another ..... Gas was probably $5 a gal. back then in Euro.....
