Subject: [Healeys] Distributors and advance

From: Chris Dimmock <[email-masked]>
Date: Sat, 21 Apr 2012 16:18:53 +1000

Hi Per,
The mechanical advance in a Lucas distributor is achieved by centrifugal
weights, the rate of movement of these 2 weights is controlled by 2 small
springs.
The mechanical advance "curve" is the answer to your question - the thing you
get "regraphed" when a distributor gets rebuilt.
Regraphing the advance curve is achieved by changing/altering springs and/or
cam. Obviously after the wear is removed by replacing worn out stuff.
The rate of advance is determined by the spring tension, and the degree of
advance is determined by the distributor Cam.
So there are only 2 things you can change.
The springs (there are literally hundreds of options - wire diameter/ number
of coils/ free length etc)
The Cam - options from 9degrees to 18 degrees from Lucas
The weights in each series of Lucas distributor (e.g. All 25d series, 4 and 6
cylinder) are all the same identical weights, there are no Lucas options to
change them lighter or heavier.
For example, specs for a 40662 type DM6 distributor typically fitted to a 3000
mk1 has these Lucas specs:
4.5 degrees at 500 rpm
13 degrees at 1,150 rpm
15 degrees at 2,000 rpm
These are distributor rpm, driven by the cam, which turns a half crankshaft
speed.
So double them for engine rpm, and this distributor will give you 30 degrees
of mechanical advance at 4,000 rpm. So total advance is static plus
mechanical, so if you have 6 degrees of static advance, you'd have 36 degrees
of advance from 4,000 rpm onwards.
At light throttle, cruising, your vacuum advance operates, increasing the
amount of advance to improve fuel economy and reduce emissions.
So. At 1,000 rpm, you will have your static advance (set by rotation of the
distributor body) plus the (4.5 degrees, x 2) = around 15 degrees.
If you graph (I.e. your advance curve) these You can work out what advance you
have at any given rpm.
Advance stops when the distributor cam hits it's bump stop, which on this cam
is 4,000 rpm. Vacuum advance stops at 8 degrees, which again is the limit of
travel of the vacuum advance mechanism.
Most vacuum at the SU throat is when the throttle is just open. Light
throttle.
Best
Chris
PS your friends car sounds more like a fuel starvation issue, or a timing
issue, if everything else is correct
Sent from my iPhone

On 21/04/2012, at 9:47 AM, Per Schoerner <[email-masked]>
> Chris
> So, at idle we have very little vacuum,
> you say that the advance at idle is mechanic?
> As engine speed rise we get more advance from the weights
>  But when does it stop?
> Is the advance from the weights proportional to the engine speed or?
> At what engine speed do the weights start to produce advance, and at what
speed do we have to most advance.
> And when do we have the highest vacuum, at half throttle perhaps?
> But while test driving his car I noticed that it didn't really work above
3000 RPM, it just misfired and was generally miserable. I began to suspect the
ignition timing
> Best regards, Per in Sweden
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