Vacuum Advance ID

[bowman]

I have a 1975 450SE and my old vacuum advance can (987 on arm) has a ripped diaphragm. I got another one that would physically fit in the space on the distributor but the arm is longer (It says 2781).

After trying this new one, I can't get the timing set correctly because the arm is longer than the original so it is rotating my base plate too far counter-clockwise at rest. I tried to compensate for this with base timing settings but I think either the spring tension is different or the simple geometry of the new set-up is throwing the swing off so I am in the weeds.

I can't find a direct replacement of my can ANYWHERE and wondered if anyone could give me advice or has a cross-reference chart with how I can source a replacement using just those last 3 numbers 987. This is frustrating because I have done a load of work to the car, this is the final piece of the puzzle and I am coming up short.

 

[ProfessorExperimental]

According to my google-fu and https://jetronic.org/de/forum/ignit...ose-am-zuendverteiler-w116-motor-450-se-bj-72 the Bosch part number for a 72 euro 450se is 1237121927 (which is Benz part A0001583018 if you're interested), so an assumption worth testing might be that your vac can us Bosch part number 1237121987. Put that into google aaaannnd:

You get a bunch of parts lists that include it, but sadly it seems to be no longer available:

https://www.autoersatzteile.de/699233-bosch

However, Bosch service should be able to provide you with a very near equivalent that you could make work (they can usually see things like the throw, pressure setting etc), or alternatively dig into aliexpress looking for a Chinese fake that'll do the job.

Or you could get creative with an adjustable blow off valve for a small aftermarket turbo install and the old (or unsuitable new) actuator... but surely there's a plunger that'll work

 

[ProfessorExperimental]

Actually, come to think if it, wtf is the actual problem here: doesn't the car run just fine with the vac line to the can disconnected and plugged so it doesn't create a vacuum leak from the manifold? It's a vacuum retard, right, for less N2 emissions at idle and reduced knock sensitivity to less rich idle mixture settings to avoid fouling plugs. If you're filling up with any decent 98RON you have fairly ping resistant fuel already and if you really want a smooth Benz idle from a 51 year old engine just tweak your idle mixture down a few tenths of a unit and rely on the upper cylinder cleaners you're paying 20c/L extra for at the pump to keep your plugs clean (along with a healthy dose of Not Sitting In Car Parks Idling For Too Long and Using All The Accelerator Travel As Gott(lieb) u. Karl Intended to Italian tune it whenever possible and safe).

If you want to keep the plumbing looking legit for concourse reasons you can always substitute a section of the plastic vacuum hardline for whipper snipper line (hdpe) of the right diameter and colour from Bunnings or 3D printer filament (pla, abs, pcg, petg, take your pick really: natural pla is probably the closest colour match, abs is probably the best material selection although white is probably "too white" to match Benz's Noodle) from e.g. Jaycar.

 

[sean sherry]

Vacumn Advance is an economy device. Car will run just fine without one. It advances the timing under light load ...cruising on the Mototway.. when the fuel mixture is set to run weaker. Weaker mixture need eirlier spsrk to burn properly. High manifold vacumn triggers the advance. Accelerate the vacumn drops... timeing is per the Mechanical. Otherwise timeing would be over advanced.... Pinging etc...
Cant get one ? economy not an issue ? leave the old one on for apperance.... Oh set the timing around 10% at idle, "no vacumn"... not 5% !!