Paul:
First, can I confirm, for comparison purposes, what your vac is actually
reading? Any engineers may wish to correct me but vacuum is measured in
inches of Mercury (" Hg) or centimeters of Mercury (cm Hg). Am I correct
that your hand held vacuum pump is reading inches of Mercury (" Hg) ?
The rest of the list:
Trevor B, suggests that the ZS carb can not be used with a 25D distributor
(or with any other vacuum advance unit) as it "...will behave oddly when
connected to the same vacuum line that the vacuum retard was. (different
vacuum signals at different times)"
My tests bear this out, but I'd like to hear from others. Background:
The stock carb for my '79 Spit with catalytic converter would be a single
Stromberg 150 CD4T. All Spitfire 1500's in North America were sold with the
Lucas 45DE4 electronic ignition and had a vacuum retard. The distributor on
my car is a 25D.
I understand that the timing should be set to 10 BTDC with the vacuum
advance unit disconnected (this is the setting for European 1500's according
to Haynes). I thought that when the vacuum advance is connected at idle the
timing should advance further. Not so in my case. Here's why: According
to my Mighty Vac, my vacuum advance unit requires 6" Hg (15 cm Hg) before
it even moves and it takes up to 17" Hg (45 cm Hg) to move the full range.
(I wouldn't mind knowing if a Lucas 45D4 or Delco Remy have differing
readings.) You ask what is my stock ZS carb sucking? I found the following
results (no load, if it makes a
difference):
Here is what I found:
RPM Hg
1000 3" or 8 cm
1700 4" or 10 cm
2000 8" or 20 cm
3500 17" or 45 cm
Thus, the carb is not generating sufficient vacuum at normal idle (650 -850
RPM) to put the vacuum advance unit in motion. This confirms what Trevor
has suggested. [BTW, the booklet with my Mighty Vac says that a gas engine
at idle usually pulls about 16" to 22" Hg. This is way above my test
result.]
I have heard from one person that connecting their single Stromberg 150 CD4T
to a vacuum advance is a good thing. There must be more owners who have
done this.
Are you all just believing it is a good thing to connect a vacuum advance to
a single ZS carb or is there some other evidence?
Len Lubbers
Ottawa, Ontario
'79 Spitfire
-----Original Message-----
From: wizardz
To: Glenn Trunnell
Date: August 15, 2000 8:59 AM
Subject: Re: Distributor vacuum advance again UPDATED - limiter plates!sorry
got a little long
edited message:
>I just switched from the single ZS to twin HS4's. So I went through the
whole ritual of testing
>every variation of vac connection., advance curve tests, and every
convolution thereof !
>
>maintain position. Using a hand held vac. pump I could watch it simply
'return to home'
>as such. As soon as I released the vac and it bleed down around 10 psi of
vac. It would just all
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