Vacuum advance ? - Mercedes-Benz Forum

 
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post #1 of 9 (permalink) Old 02-16-2012, 06:27 PM Thread Starter
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Vacuum advance ?

The retard in my vacuum advance is unhooked , should I try and hook it back up, most of the valves that these vacuum lines are unhooked and the advance on the distributor is just hooked to the carb.
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post #2 of 9 (permalink) Old 03-18-2012, 01:17 PM
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Hello,

I would attempt connecting things the right way, if you are lucky the RPM relay and SOVs are still working. Idle timing should be around 4 deg ADTC with retard. Check for max advance at 3000 RPM.

The ports on those carbs supply only manifold vacuum, so connecting one straight to the distributor advance really messes up the timing curve. On my '70, I had to repair the RPM relay (old leaking capacitors) and replace both SOVs to get everything working correctly.

Last edited by Leone; 03-18-2012 at 01:47 PM.
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post #3 of 9 (permalink) Old 03-18-2012, 02:20 PM Thread Starter
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I thought about hooking it back up or trying a earlier distributor with out the vacuum retard.

'71 250C "work in progress"
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post #4 of 9 (permalink) Old 03-18-2012, 04:06 PM
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Originally Posted by r34sklnnsn View Post
I thought about hooking it back up or trying a earlier distributor with out the vacuum retard.
The later Zeniths are a pain, without a source for ported vacuum you have to rely on the RPM relay and SOV to work the vacuum advance. Some folks leave the retard disconnected, but without the idle retard you might find that you run out of adjustment on the carbs and can't bring the idle speed down within spec.
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post #5 of 9 (permalink) Old 03-18-2012, 04:24 PM Thread Starter
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Sorry, should have put that my carbs have been switched over to Weber's so they don't have the same hookups as the factory Zeniths.

'71 250C "work in progress"
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post #6 of 9 (permalink) Old 03-18-2012, 05:50 PM
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In that case, less hassle for you, you can forget the relay and SOVs. There's no real need to run the vacuum retard, although it does reduce idle emissions (and exhaust reek) somewhat.

Book spec with retard is 4 deg ATDC, and the retard pulls the timing down about 10 degrees. So without retard, figure 5-10 deg BTDC at idle, check for max advance (31-39 at 3000 RPM) and you're golden.
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post #7 of 9 (permalink) Old 03-18-2012, 05:53 PM Thread Starter
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Thanks for the help.

'71 250C "work in progress"
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post #8 of 9 (permalink) Old 03-19-2012, 08:17 AM
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Just for fun pull the distrib and have the advance calibrated on the machine, any hot rod shop will have a distributor person.
Leone's advice is correct.
A $20 Bosch/VW vacuum unit will fit the older MBZ distributor.
Do clean and lube the advance weights.

Essentially, you will take it back to the pre-smog engine timing/economy, proper advance.
Those retarded low compression smog configurations killed fuel mileage. I could never figure out how you could produce less pollutants by burning more fuel.

I loved the full vacuum advance and 10:1 compression ratios with 100+ Octane . large engines (427ci Hemi 1967)

Brian O
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post #9 of 9 (permalink) Old 03-19-2012, 04:33 PM Thread Starter
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That is good advise, I checked the advance today and it is sticky so I will take it apart and I talked to a friend at work that use to dirt track race and his old engine builder will calibrate it cheap. I'm in no rush sense it runs great and after I finish a top end rebuild on my brothers Rodeo I'll start tearing it down for a respray .

'71 250C "work in progress"
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