Childs, David wrote:
>
> I have a DGV with the same system. I have the same questions! I
> drilled and taped a port in the intake manifold and was happy with the
> performance. The idle is better and top end is good. I think that
> it all has to do with vacuum advance or vacuum retard on the
> distributor. I don't know about cars in the UK but I think most of
> the 79 Spits in the US have vacuum advance. Unlike the 75 Spit that
> had the retard. A list member suggested a way of determining if your
> system is advance or retard. Look at the direction that the vacuum
> port is pointing on the distributor. If it is pointing away then
> advance, if pointing in then retard. I believe the 79 Spit has the
> electronic ignition that is not attached to the distributor. This
> system, I think, is advance.
>
> The info that I need now is, what does the mechanical advance curve
> look like, and what should the vacuum + mechanical curve look like?
> If we had this information we could solve the mystery!
>
> Dave C
> 79 Spit
>
> ----------
> From: James [SMTP:james.carpenter@ukaea.org.uk]
> Sent: Friday, August 07, 1998 7:08 AM
> To: Spit Fires
> Subject: Vacume Advance Opperation
>
> Here is a puzzeler that I don't understand, to do with the vacume
> system
> on the UK Spitfires, and most car's.
>
> The vacume advance is connected to a tap point on the rear carb.
> This
> apperes to be the wrong side of the butterfly. I put my vacume
> metere
> on this tap to see what's happening.
>
> At idle you get no vacume, you only start to get a vacume when the
> throughtle is opened a littel. This can be explained by the position
> of
> the tap, but why is it designed to do this?
>
> --
> James Carpenter
> Yellow '79 spit wired by a trained marmot
I am not sure if this will shed some light on UK Spitfires but this is
what I have learned on the US Spitfire (Particularly the 74 Spit).
On the 74 US Spitfire, there were 2 vacuum advance Ports. Actually on
the Delco Distributor that I am using, the unit is actual a vacuum
retard unit, but this is not important (really). The 74 Spits used a
temperature controled vacuum valve. This valve changes the location in
which the distributor obtained its vacuum. Aparently when the coolant
temperature is "cold", the vacuum tapped off one side of the butterfly
valve and when "warm" tapped off the other side. I am not certain which
is which, but one thing is for certain, it only affects idle. The
vacuum pressure is essentally the same on both sides of the butterfly
valve after it has rotated a certain amount. A buddy at work told me to
forget the temperature valve, and hook the vacuum advance(or retard, in
this case) to which ever side of the butterfly valve made the car idle
better.
--
Patrick Barber
74 Spitfire 1500 (FM14774U)
"Go Red Wings" ... 1998 Stanley Cup Champions!!!
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