Distributor Vacuum Retard

Steven Mechanic Washington Posted   Latest  
Resolved
Driveability
Emissions
1978 Mercedes-Benz 450SLC 4.5L 3-spd (722.0)

Question for older techs. I set the ignition timing to the spec of TDC (zero degrees). Then with the vacuum lines put back on the distributor, the vacuum retard pulls the timing back to 5 degrees ATDC. Does this sound right? Did cars back then with this emission control feature have timing that was ATDC instead of BTDC at idle?

The spec with vacuum hoses connected is about 8 degrees BTDC, so something is wrong. One of the specs is wrong, or the listed specs are for a car that didn't have vacuum retard. What's been your experience with cars that had vacuum retard?

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Albin Diagnostician
Washington
Albin
 

Where is Obie when you need him? I remember having a 1970 Toyota highlux pickup with 8RC engine. That engine used a double diaphram advance/retard device on the distributor. I don't remember a lot about it, since it has been over 30 years since I worked on it last. It seems to me that at certain engine temperatures that would retard the ignition timing at idle. As best as I can remember, that…

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Troy Owner/Technician
Washington
Troy
 

if I recall it is correct it should advance once off idle

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Olle Instructor
Minnesota
Olle
 

If you let it warm up, does it drop back to 8 BTDC? I believe it is supposed to.

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Chad Owner/Technician
Colorado
Chad
 

After reading several forums post about MB cars this vintage it seems like the exact spec carries slightly depending on engine number and distributor numbers. the most common information I found mentioned 0/TDC with vacuum connected and 5 ATDC with vacuum hose connected. Most forum members mention diminished performance if set to the book and instead they bump it up slightly until better…

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Eric Instructor
California
Eric
 

Sounds about right. There is probably a thermal vacuum switch involved that will route the vacuum to the advance diaphragm one a certain coolant temp is reached. They were trying to warm up the cat or thermal reactor after cold start.

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Victor Owner/Technician
Florida
Victor
 

You need to identify your vacuum source(intake or ported)

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Martin Owner/Technician
New York
Martin
 

Kinda brings back memories though I am not that old. There are a number of things at play here. A thermal vacuum switch can make a difference as well as your source vacuum which Victor just mentioned. Look at the position of the vacuum plunger arm on the breaker plate or essentially the circumference of the distributor, advance will pull the arm and into the direction of rotation. So the…

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Troy Owner/Technician
Washington
Troy
 

74 should be 5ATDC but 75-78 0 TDC so I would bet chain stretch has your timing off it is to be set lines connected at 800 RPM

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Steven Mechanic
Washington
Steven Update
 

Thanks for the help guys. I will be getting back to this car sometime today most likely, and I'll update at that time.

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Frank Service Manager
Nevada
Frank
   

Here is a link, free repair manual for this chassis (107) startekinfo​.​com/StarTek/doclis…​.​jsp startekinfo​.​com/StarTek/outsid…​.​pdf

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Steven Mechanic
Washington
Steven
 

Hello Frank, Yes, I was using that free online resource, but whenever I clicked on the links to the ignition timing, it would just link to the same page I was already on. Did you do have to do something to get the links to work, or did they just work for you? Some links worked fine, but none of the ignition stuff would link properly for me.

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Frank Service Manager
Nevada
Frank
 

No problem on my end opening links

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David Owner/Technician
Ontario
David
 

Thats correct. Centrifugal advance, advances based on rpm. Vacuum advance retards it with vacuum when engine load is low and allows it to advance when your load is high because vac drops. Many systems used ported vaccuum from the carb or some sort of temp controlled changes and switching to take place. Generally speaking but cant say for sure what Mercedes did.

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Steven Mechanic
Washington
Steven Resolution
 

What I ended up doing is setting the total advance with both vacuum hoses disconnected and plugged. I set it to 35 degrees total at 3000 rpms, and then reconnected the vacuum hoses. I also tuned the air/fuel mixture and idle speed, at the same time so that all three settings were good to go. Took it for a road test and it runs just fine. I might do some fine tuning depending on how it starts and…

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Troy Owner/Technician
Washington
Troy
 

If you dont mind me asking what was the timing when you were done?

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Steven Mechanic
Washington
Steven
 

At idle it was about 10 degrees BTDC without the vacuum hoses connected, and about 5 degrees BTDC with the vacuum hoses connected.

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