On 27 Jun 2002 at 21:17, Graham McMicken wrote:
> It is not advancing at all.
You mean that you can't turn the rotor by hand? It sounds like your
advance mechanism is stuck. As Randall wrote, you should be able to
take it apart and clean it up.
> Could you please elaborate on the vacuum retard unit?
> I'm not sure what it does.
The retard unit is an idle-only thing. The vacuum tap for it comes
off of a carb just behind the throttle plate. When you open the
throttle even just a little, the opening is exposed to the upstream
(non-vacuum) side of the air stream, so the vacuum goes away. On the
dizzy the arrangement is such that when vacuum is present it pulls
the timing late by quite a bit, perhaps 15 deg or more. (The actual
amount is the difference between static and idle timing.) As with
any sort of vacuum unit, the vacuum diaphram itself could be arranged
such that it pulls or pushes the plate holding the points, and the
vacuum line could be on either side of the diaphram. Whether the
unit acts as a retard or an advance depends on both features, which
is to say, on its net effect.
Its sole purpose is to help clean up idle emissions. You can
disconnect it and plug the line and not affect the normal running.
But you'll have to rest the idle speed because the idle will climb.
Also it does make a difference in idle emissions. When my Spitfire
was still subject to emissions testing I found that I could more
reliably set the timing at idle and it could be made to idle smoother
with it disconnected, but it would fail emissions that way. So I
kept it connected. Still do, if I recall. Haven't looked at it on a
long time!
--
Jim Muller
jimmuller@pop.rcn.com
'80 Spitfire, '70 GT6+
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