Ah yes, the old "vacuum retard" issue again.
You can't really hook up the vacuum advance of your MK III distributor to your
MK IV Stromberg carb.
The problem is that the Stromberg's vacuum port provides a vacuum suitable only
for the MK IV distributor's
vacuum retard unit. It provides full vacuum at idle, and no vacuum at part or
full throttle. On the stock MK IV,
you will get retarded timing at idle strictly for emissions purposes, and no
vacuum advance
during actual driving.
A conventional "ported vacuum advance" setup like the MK III used provides no
vacuum at idle,
partial vacuum at partial throttle, and no vacuum at full-throttle. This
causes no vacuum advance
at idle (for emissions) and maximum advance at partial throttle (for economy).
Your best bet is to simply leave the vacuum advance unconnected, and set the
timing to 6 deg. BTDC or so.
When you are not using a MK IV vacuum retard distributor, you MUST IGNORE the
stuff
about the 2 deg. ATDC timing!
Or you can get some MK III SU carbs to go with the distributor :-)
The same problem arises in reverse when you attempt to use SUs or Webers
with the stock MK IV distributor.
Doug Braun
'72 Spit
At 08:48 PM 7/13/2004 -0400, you wrote:
>Friends -
>
>When I rebuilt the engine in my MkIV recently, I replaced the worn-out
>distributor with a unit I got from a junker, a 1971 Mk 3. The replacement unit
>had been recently rebuilt, less than 500 miles when the car was rear-ended.
>It's in good shape. My mechanic, the estimable Dave Crane, liked the unit and
>said the advance would be an improvement since I had the engine rebuilt to
>stock configuration (no special cams or anything).
>
>The Delco-Remy number on the side is 7593557. That "3" may be an "8", it's
>very faint. It has the vacuum advance unit on the side plus the adjustment
>knob that lets you advance or retard the spark while the engine is running.
>
>So, here's my question - anyone have any idea of where to set the ignition? My
>Bentley manual mentions several Delco distributors by number, but not the
>number above. Right now I have it set to 6 degrees ATDC static which comes out
>to 2 degrees ATDC at idle. Advance seems good, I can rev the engine with the
>strobe light on and the spark advances smartly.
>
>Engine seems to run a little hot - at least the gauge reads higher than it
>used to. I understand that new engines tend to run a little on the hot side,
>but I wanted to get a second opinion on that ignition timing.
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