Thanks Paul. Good advice. Some of the 'California'
cars went through very strange set-ups as tehy
tried to meet ever changing emissions standards.
As you comment, there are three vacuum schemes;
(1) Manifold vacuum from a port ON the manifold.
+ High at idle
+ Low at WOT accelerating
+ Very high on over-run
(2) Carb vacuum from an UN-blanked port(where
butterfly does not cover the vacuum hole
in the carb throat but is BELOW butterfly.
...sames as (1) above
(3) Carb vacuum from a blanked port (where
butterfly covers or 'blanks' the vacuum hole
in the carb throat.
+ Low at idle
+ Low at WOT accelerating
+ Very high on over-run
How the timing and distributor are set up should
depend upon the way the vacuum is created.
best regards,
rick
>
> The book figures always erred on the side of conservatism even when new to
> take account of manufacturing tolerances and individual cars could often
> benefit from slightly more advance than the book figure. 30-odd years
down
> the line, especially with different components to that originally, and
> petrol nothing like it was then, the book figures are even less relevant
> although they can be a useful starting point. The centrifugal and vacuum
> curves are probably nowhere near the curves that would be ideal for your
> engine now, possibly only touching in one place like a cheap suit :o)
> Basically run it with as much advance as it can take without pinking at
any
> combination of revs throttle opening and load and that will take account
of
> all the variable of your engine. 10 degrees BTDC sounds about right, I
> don't know where 2 degrees after comes from, once centrifugal and vacuum
> advance come into play you get more advance, not less, and can be up to 50
> degrees.
>
> I don't know about the Midget but the MGB always used vacuum advance, and
> this was irrespective of whether the vacuum source was the carb or the
> manifold, despite what some may claim. The difference was only an
emissions
> frig and any system can be run with either. In fact when the UK engines
> changed from carb to manifold vacuum they retained the same engine and
> distributor. Generally performance is reckoned to better with
pre-emissions
> distributors and settings.
>
> PaulH.
>
> ----- Original Message -----
> From: "Rick Lindsay" <rick@stoolhead.com>
> To: "mgs" <mgs@autox.team.net>
> Sent: Tuesday, June 08, 2004 4:51 PM
> Subject: Midget 1500 ignition timing
>
>
> > Where am I going with this? I need is to design
> > the 'best' static or low engine speed timing for
> > general street use. The manuals vary between 10
> > degrees BTDC to 2 degrees ATDC. These numbers
> > represent both emissions-related settings AND the
> > way the vacuum is ported to the distributor.
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