Not so, this is an oft repeated error. Apart from at idle and fractionally
off it the resulting vacuum advance is identical through the throttle range.
The carb port is not outside butterfly (that is where the manifold port is)
but under the edge of the butterfly when it is at idle where it gets
negligible vacuum. But as soon as the butterfly opens a little way, no more
than its thickness, the port is exposed to the full manifold vacuum outside
the carb i.e. where the manifold port is. I did my own tests switching
between the two and documented the results, showing that the throttle only
has to be opened a very small amount, much less than cruising position, for
the vacuum to be the same, see
http://www.mgb-stuff.org.uk/wn_ignitionframe.htm and click on 'Vacuum
Advance - Carb vs Manifold'. At anything more than moderate acceleration
there is no vacuum advance from either to prevent pinking with the higher
combustion pressures, it is only there to give improved cruising mileage
although it does sharpen part-throttle acceleration as well. UK cars only
moved from carb to manifold in 77, but kept the same engine and distributor
when they did so and for the whole of rubber bumper production. American
engines, carbs and distributors only changed so frequently because of the
continual running battle to stay within the steadily reducing pollution
limits. The only reason it changed anyway is that the higher advance at
idle results in a higher rpm for a given throttle opening, so the idle can
be turned down a bit, reducing fuel consumption and hence pollution. The
irony is that eventually American cars got so much anti-pollution stuff that
it started causing running problems, so vacuum advance had to be limited to
4th gear only, and in some cases disabled altogether.
PaulH.
.
----- Original Message -----
> MG distributors used two methods of ignition (timing)
> control. One is manifold vacuum controlled - off the intake
> manifold, the other is 'ported' - the take off is just
> outside the throttle butterfly. Mix up the two and your car
> won't run right.
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