L&B Lubbers wrote:
> 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)"
Just to clarify, I am not saying that the ZS carb can not be
used with a 25D distributor. Just that you can't use them stock
by hooking up the same wires, you have to rethink a few things.
If these are the bits you have, there is no reason you can't
make them work satisfactorily with a few head scratchings and
some thought.
> I thought that when the vacuum advance is connected at idle the
> timing should advance further. Not so in my case.
You are digging into the reasons as to why the vacuum signals
are "different".
There are two main classes of vacuum signals used to
control distributors.
One is manifold vacuum, which just measures the vacuum
in the intake manifold. It's highest at idle and at
great load, and relatively low at cruise.
The other is ported vacuum. Ported vacuum has the port
positioned so what it senses actually depends on the
throttle butterfly position.
There are a few types, but one you see often is situation
so that vacuum is more or less zero at idle, but increases
with throttle opening and various loading conditions.
Without going into details, just understand that
graphs of vacuum signals over engine range can be
very varied and far from straight lines.
So, there isn't always a linear relationship between
vacuum and RPM. As mentioned, ported vacuum starts
at nothing, then goes high, then gradually goes
down to low again as you rev it up.
Since your manifold is made for ZS, it's almost guaranteed
to be manifold vacuum which is HIGH at idle and fades
to zero as revs increase. It's sort of the same for a lot
of the range but VERY off at idle. (remember how this
convo started, why are late spitfires supposed to
be timed really strange 4ATDC at idle, it's all coming
into the puzzle).
> 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?
Go without vacuum. control of any kind for a while,
plug all the ports. Vacuum signals fine tune performance
under many situations, but performance is generally not
one.
Usually it just gives you lower emissions, or smoother
idle, etc.
--
Trevor Boicey, P. Eng.
Ottawa, Canada, tboicey@brit.ca
ICQ #17432933 http://www.brit.ca/~tboicey/
"Bury me alive, I'll give you a dollar!" - Johnny Bravo
|