OK, I just bought this NEW vacuum advance/retard unit on Ebay for my '72 250 (mainly because it went cheap). But, now I'm confused. The vacuum advance side (the mushroom side) has a little pin size hole right in the center of the mushroom and will not hold a constant vacuum when I suck on the inlet - when I cover the hole with my finger it holds the vacuum just fine. The retard side hols vacuum. I have an older 'back-up' distributor with the same vacuum unit and it looks like it has the same hole, but it holds vacuum. (My wife thinks the old one is just clogged??) The little holes look like they are supposed to be there - does anyone know for sure? I haven't removed the one from my car yet, but it also looks like it has the same pin size hole in the center.

Question: Is this hole supposed to be there? According to the manual, if engine speed increases above 2400 rpm in the temp range of 17C - 100C, vacuum moves to this advanced side of the box. Maybe the vacuum side is not supposed to hold a constant vacuum, but is supposed to bleed off slowly? Can anyone explain this to me? Thanks.

Ryan

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RG Newell

1984 300D
1972 250
1986 560SL
1991 300CE

they make three of these. One for advance, one for retard and one for both. The ones that look visually identical should work the same way. The new one works right. The old one doesn't. Or did I just answer the wrong question. -CTH

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- cth350@gmail.com
http://www.111lakeside.com/

Sorry CTH, but I didn't follow your response (probably because my question was confusing). I have a distributor vacuum advance that BOTH advances and retards the timing under the appropriate conditions (coolant temp, rpms). My question was does my old unit work right or does the new one? They both look like they have small pinholes in the center of the advnace side, but where the old one holds vacuum, the new one does not (unless I cover the hole). Does that make sense?

Ryan

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RG Newell

1984 300D
1972 250
1986 560SL
1991 300CE

I am going to visit my '72 tomorrow and will look at what it has on the 'pot. There must be a reason for the hole - the only one I can think of is to modulate the effect of the vacuum on the movement of the lever and the advance mechanism. The real test is if it does what the book says it should with the timing.

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Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe

Chuck,

I checked with the seller of the vacuum box (he has listed another brand new unit on Ebay) and his other new one performs just like mine. I am assuming that there is supposed to be a hole and that the vacuum bleeds off through this hole. I guess this makes some sense because best I can tell, this advance only works when the car is between 17C - 100C and rpms increase above 2400. Does that make sense to you?

Ryan

__________________
RG Newell

1984 300D
1972 250
1986 560SL
1991 300CE

I think that the whole theory of the advance - retard was that retarding the timing at lower RPM's and when the engine was warming up would decrease emissions. So when it hits 17 C and cruising RPM, they allow more advance. The 100 C limit is to maybe prevent overheating, since advancing makes it run hotter.

__________________
Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe