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Old 06-11-2006, 06:53 PM
Bondavi's Avatar
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Join Date: Feb 2005
Posts: 316
Timing moves off TDC even with vacuum advance disconnected

On a 1983 500SL

My car has a distributor with advance only, no retard. Specs are for 16 degrees BTDC at idle, 25 degrees at 3000 rpm. While idle is fine, I'm seeing 35-40 degrees even BEFORE 3000 rpm!

I disconnected vacuum advance from my distributor to check a vacuum problem I'm having at higher RPM's. At idle, the timing gun shows TDC, but as I apply more revs, the timing mark MOVES as well, towards more degrees BTDC, about 10-15 degrees. So this is why at rpms the advance is way too far off, something is compounding on top of the advance curve.

Looks like something is advancing the timing EVEN with advance totally disconnected. What could be doing this? Is this normal? I would have throught it would STAY at TDC throughout rpms when vacuum was disconnected.

What could be causing this??

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Old 06-11-2006, 07:39 PM
Bondavi's Avatar
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Join Date: Feb 2005
Posts: 316
Pulled the cap and the rotor has an spring loaded play in it in one direction. The timing advance ring is separate from this and is functional.

Is there supposed to be zero play in the distributor shaft, or is it spring loaded? Related with dwell? I would have thought there'd be zero play on the shaft.

If anyt of this is normal or not, I'd like to know. Thanks!
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Old 06-12-2006, 10:45 AM
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Join Date: Sep 2001
Location: central Texas
Posts: 17,281
I have no manuals on anything except Diesel MB's... so I am just going from 35 years of mechanicing ( ie, generic guessing ) ... I would find it very unusual for a car to have only vacuum advance. It probably has a combination of centrifugal advance and vacuum...and your symptoms indicate the calibrated spring on the centrifugal advance has given up the ghost...

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