Here is the answer I got from Jeff at Advanced Distributors when I asked
him why he converts a DM2 distributor with a 25D mechanism:
Mike,
The change in points is a very small portion of the repair. Actually that
would only make for less than 10% of the performance improvement. The
biggest improvement is made in the way the advance springs hold tension at
rest. With the original style advance, the rate at which tension is applied
to the springs is a slowly increasing curve, which only applies very light
spring pressure at low advance rates. The 25D style advance assembly
situated the weights and springs pins quite differently. The springs apply
pressure at a very linear rate from the first degree of advance throughout
the entire advance. At low advance rates, the 25D assembly has the spring
pressure to return the weights to the "closed" or fully retarded position
with ease. Even a properly assembled and lubricated DM2 advance assembly
doesn't have the spring strength to do that, so you will get low speed (low
advance) timing variations of 2-5 degrees.
You can see in the attached picture a rebuilt 25D advance assembly on the
left, and the old DM2 assembly on the right. You can actually see how the
two aluminum advance weights on the right are spaced apart, meaning they
don't return to the closed position, rather they hold advanced about 3-4
degrees. That's because the double-fulcrum design of the "toggle" advance
system doesn't apply enough spring pressure at the angles in which its
situated. If you look at the 25D assembly, you can see the direct action
that the advance springs will take to reduce advance and "close" the
assembly.
This is a modification that can be made to virtually all pre-'61 Lucas DM2
distributors to improve performance and reliability. I commonly make this
upgrade to MGAs, TR3s, Magnettes, and others. The BN1 is only one
application.
Jeff
Jeff Schlemmer
Advanced Distributors, LLC
612-804-5543
jschlemmer1@comcast.net
I can forward a copy of the picture to anyone interested at their personal
email address.
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Healeys@autox.team.net
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