Strictly speaking the mechanical and vacuum advance characteristics of the new
distributor should match the requirements of the engine. On the face of it
that is the same as the original distributor, but for most MGBs designed for
leaded petrol the characteristics required now can be very different from the
original, and the octane of fuel used makes a huge difference. Mechanical
advance varies with revs and so is expressed as a set of figures, vacuum
advance varies primarily with throttle opening and is expressed as a min and
max. The purpose of the curves is ideally to keep the advance just short of
pinking at any combination of throttle opening, revs and load, for best
performance and economy. But being mechanical they were only ever crude at
the best of times. Add to that the situation from 67 on where meeting
emissions limits became more of an issue than performance and economy, and you
end up today where really the only thing you can do is run with the most
advance you can short of pinking at the foresaid any combination of throttle,
revs and load. But even then with low compression engines it is possible to
put so much advance in it stalls the starter, so that is another criteria. As
far as connection of the vacuum is concerned carb is better as there is no
advance when cranking for easier starting, which there probably will be with
manifold advance which can make starting a bit harder. Other than cranking
and at idle there is no difference in the vacuum signal, you only have to open
the throttle a little way for the two signals to be the same. If you have
carb(s) with a vacuum advance port then use it, otherwise you will have to use
a manifold port. Any unused port must be sealed to prevent vacuum leaks.
PaulH.
----- Original Message -----
I recently installed a Pertronix Flame Thrower distributor in my 71 MGB GT.
http://www.mossmotors.com/SiteGraphics/Pages/flamethrower/flamethrower.ht
ml
For proper ignition timing purposes, would anyone be abIe to provide
the following information:
1 - Distributor mechanical advance of this distributor: 10 degrees?
or 20 degrees? or other?
2 - Vacuum advance: connection to manifold? or carburettor?
3 - Recommended Ignition Timing, based on the above and engine year?
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