[Mgs] anemic running B

[Paul Hunt]

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.