[Mgs] Pertronix Flamethrower and vac advance

[PaulHunt73]

Distributors for the MGB were not 'designed' for either vacuum source, they are identical.  The only difference between the two sources is at idle - carb vacuum gives zero vacuum at idle and manifold vacuum gives near maximum vacuum.  As you start to open the throttle the carb vacuum source rises rapidly to meet that of the manifold vacuum source, and at all normal driving positions of the throttle they are the same.  A comparison of the two can be found here http://www.mgb-stuff.org.uk/ignitiontext.htm#vac

The change was done to reduce emissions at idle as with more advance you can reduce the carb idle screws and so use less fuel and hence have less emissions.  It's true that North American spec cars had many changes of distributor, but that was all about trying to met the ever tightening emissions regulations.  UK cars kept the same carbs and distributor throughout rubber bumper production, even though the vacuum source moved from carb to manifold in September 76.

If you have an HS with the port then use it, and make sure any manifold port is plugged.  But if not then use a manifold port.  Either vacuum results in better part-throttle acceleration and gives better cruising mileage.

PaulH.

  ----- Original Message ----- 
  ...Since then, I've learned that those distributors are designed to use ported vacuum and not manifold vacuum. Am I getting any benefit at all from having the vac advance hooked up to the manifold? Should I disconnect it and cap the intake vacuum port. Or, should I find an HS4 carb body that's equipped with the vacuum port?
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