Fine tuning

Now that the car is somewhat drivable, I can pay attention to getting the engine running well.  First step was to install a Pertronix electronic ignition kit to replace the dual-points in the distributor.  I’ve used these kits in previous cars with excellent results, so I was happy to see a kit (model 2865) exists specially for the M189 and it’s dual-point distributor.  Along with the pertronix, I changed to a Bosch Red coil (A 000 158 49 03) and 1.8ohm ballast resistor (A 000 158 17 45).  New wires and rotor are on order, but did’t arrive yet.  I won’t replace the cap due to the high cost, but the one on the car looks like its in good shape.

With the new ignition parts the engine ran better, but still not well.  I checked the cold start valve for leaks by removing the small bolt on it, as described on the old 300SE.org site.  No fuel came out after some time, so the valve doesn’t appear to be the source of the rich running.  I then disconnected the throttle linkage with the engine running and operated only the butterfly on the intake manifold.  The engine reved up quite a bit, confirming that its running very rich.  I shut it off and let it cool down.

Next step was to adjust the valves.  I’ve heard its possible for the intake valves to recede into the head quite badly over time, so I didn’t want to run the engine much more without checking them.  All the valves were tight, but not terribly so.  Adjustment was tedious, but I finished them all with the help of a friend in about 30 minutes.  A 14mm crows foot and good quality screw driver are a must for this job.

Now back to the fuel system.  According to several postings on the M100 message boards, it seemed likely that the rack in the injection pump is hung up from old fuel varnish that developed after years of sitting.  To free it, I used a bulb syringe to push acetone into the injection pump through the fuel line.  I used an extra fuel line that I ordered that normally goes on the output of the electric fuel pump, A 111 476 09 26.  This fit to the input connection of the injection pump and make it quite easy to fill it with acetone.

After soaking it a bit, I removed the small cover at the front of the pump to access the end of the rack.  This is a very difficult job because of its location.  Removing the distributor made it a bit easier, but a collection of long tools was needed to get in there.  I screwed a long M5 bolt into the end of the rack and check it for movement.  It move fairly easily, but didn’t return to its center position on its own until I moved it several more times.  Then I took this video to show its movement:

You can see it moves quite nicely here.  I put everything back together and ran the engine again.  Big improvement!  The engine reved up very easily now and idled much smoother.  However, once it warmed up, it would stall at idle again.  I think the cold start valve might still be an issue, so I’ll order up the seals and take it apart this week.  Just to be sure the injection pump rack can move freely, I filled it with acetone again to let it soak over the next few days.  This certainly can’t hurt!

2 thoughts on “Fine tuning

  1. Congratulations on this impressive progress!! allow me to add two cents of experience regarding the Pertronix system: usually, when they fail, they fail completely, everybody tells me. But not always! As I now know, mine started to deliver faulty signals which lead to heavy misfiring/backfiring.
    After chasing several other issues (rust/dirt in the carbs? mixture too lean? incorrect jets? floater level? etc etc) and a spectacular valve cover seal failure at 60mph (think formual 1 engine failure) followed by the classic “replace everything step by step” routine, the Pertronix sensor was finally identified as being the cause. it was five years old, and yes of course it was the last item we checked. Car now has points again and purrs like a kitten.

  2. The rear bags sound like a real chore compared to the W112. Mine looked identical but I don’t recall having to do anything other than remove the whole assembly, nothing on the suspension had to be moved. I am sure of that because it only took me about an hour to do each side.

    Nice ride – I always liked this vintage MB. Hopefully a nice Finny is in my future some day….

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