I've long suspected that my 6.9 wasn't performing quite as well as it should be. Last week I had the good fortune to attend the Fall "Lodefest" in Oregon. Lodefest is an event organized by the M100 group which covers the 300sel 6.3, the massive 600 and the 6.9. There I had the chance to swap 6.9's with somebody so we could compare notes. This confirmed my suspicions - my euro 6.9 was certainly no quicker than the other guy's USA version, and possibly even a bit slower (subjective judgement based on seat-of-the-pants driving on various mountain roads).
Fortunately Mike Morris of Morris Motors https://www.morrismotorsusa.com/
, regarded by many as a demi-god (or more!) in the world of old Benzes, was in attendance so we were able to discuss the issue. Mike was adamant that something was going on with the distributor/timing and to do nothing else until that was sorted out 10000%. I looks like he was 10000% correct. My timing was kind of right, in a ballpark kind of way, but still funny. Sometimes the idle would spontaneously change a little. Sometimes idle speed would hold steady when AC was switched on, other times it would drop. Sometimes the vacuum advance at high rpms would move around. So, I decided to take the step of taking the whole damn distributor apart and looking for problems. And I found some! Going through the process would have been a lot easier if I had "how to rebuild your distributor" guide, so I figured it might be helpful to document it all. What follows is a guide to dismantling the distributor and what I found when I did. Here goes.The Process:
Step 1: Remove the distributor, duh. When you take the cap off be certain to mark the position of the rotor so you're in the right place when you put it back together. Otherwise you may go nuts, either from trial and error or trying to crank the engine to find the TDC reference. Plate 1: distributor as removed from car. These distributors have 2 compartments. The upper contains the ignition mechanism communicating with the coil (stator magnet and rotor) and the vacuum advance/retard mechanism. The lower compartment contains the centrifugal advance mechanism.
Step 2: Remove vacuum advance pod and cable connector. For the vacuum pod remove the screws and carefully disengage the arm from its pin inside the distributor. The pin faces down, so gently wiggle the arm off in a downwards direction. Note that the vacuum pod has 2 small plastic parts on either side of the shaft (Plate 2). These are fragile but can be wiggled out. Don't try to force! The cable pulls straight of the distributor body.
Step 3: Now let’s examine what we’ve got and how it’s assembled. The components are labeled in Plate 3. The base plate is fixed to the distributor body by 3 hex screws (3mm heads). The coil is fixed to the base plate from underneath. The stator (magnet) rotates about 30deg on the central shaft. In my case the stator rotation was impossibly stiff and didn't rotate freely at all.
On the bottom side of the stator is the pin connecting to the vacuum advance pod. this pin limits the rotation of the stator magnet. The rotor is held on the shaft with a c-clip and index pin (see plate 4).
Step 4: To disassemble first remove the c-clip and spring washer on the shaft. You can then remove the rotor, though it may take some effort. Note that there's a miniscule alignment pin that locates the rotor on the shaft. Don't lose it! Next remove the entire base/coil/stator assembly. First remove the c-clip and washer so the assembly id free to slide off the shaft. Then remove the hex screws fixing the base plate to the distributor body. It’s fiddley but can be done (see plate 5). Set the assembly aside and have a look at the now exposed centrifugal advance components (see plate 7).
Step 5: Yikes! All the centrifugal elements were gunked up! The brown is gum and varnish and maybe a tiny bit of rust. The action was sticky and sluggish. The parts are shown in Plate 7 as they were found, not a pretty sight. Removal of the weights, springs etc is straightforward. They are shown in Plate 8. I blasted everything clean using ground coconut shell as the blast medium followed by air blasting and solvent wash. Plate 9 shows everything cleaned up. Pretty nice, eh? The action was then perfectly smooth and immediate. It's worth noting that when you lubricate the wick under the distributor rotor, this is where that oil ends up. It's not surprising that it turns to gum over the decades and literally gums up the works. I'd hazard a guess that many, or most, of these older cars have something similar lurking in the same place.
Step 6: Now let’s return to the upper section. The coil needs to be freed from the bottom plate to remove the stator magnet. This is accomplished by removing the 3 flat head screws on the underside of the bottom plate (see Plate 6). These screws are extremely tight! I had to use very strong downward pressure to not strip the screw heads (which were perfect beforehand and are now a bit scarred). The stator coil and magnet can then be removed. Plate 10 shows the bottom plate separated from the stator magnet. Note that the magnet plate needs to be able to move freely on the bearing on the bottom plate. Mine did not, it too was gunked up. After cleaning it up the stator magnet rotated freely. For the vacuum advance to work right the stator magnet must
be able to rotate freely.
Step 7: Reassemble. A couple tips:
1. Keep the distributor in an upright position (like in a vise) and do not pull up on the central shaft
until everything is put back together. Doing so may unseat the springs.
2. The stator/bottom plate/coil assembly can be reassembled and installed as a unit. Dropping the
hex screws into the hole in the bottom plate before installation makes this easier.
Step 8: check and set timing. This has been discussed in depth elsewhere so I won't repeat it.The results:
- Idle speed stays rock-solid whether AC is on or off.
- Timing could be set exactly per the specs and it seems stable.
- Performance was significantly improved!! I had previously recorded 0-60 mph times around 10.5 seconds. Results are a bit inconsistent cause I'm using a GPS phone app and it just never gets exactly the same number twice, but still gives an idea. New 0-60 time was 8.5 seconds, and the improvement was very seat-of-the-pants noticeable. There was no knocking or pinging. It's worth noting that the 8.5 sec number was at 5000 ft elevation and on a very mild upgrade (closest place I could test near my house). At sea level the improvement would doubtless be more. I could clearly feel the difference between driving in Oregon (~1200 ft) vs locally.
My timing setting above was really a first approximation - somewhere within spec, but the specs are a range and I didn't pay too much attention to exactly where. Since there's no knocking I'll see if I can squeeze out a little more by a hair more advance. What needs to be stressed here, I think, is that there really is such a thing as "close," and just close isn't good enough. In addition, the problems were totally invisible until the distributor was taken apart.
In closing, this project brought major performance gains at no cost and with no parts. If you've got any symptoms that this might explain then go for it! There's nothing to lose, much to gain. Happy motoring and cheers,