[xk] Distributor Advance Curce Tester Made from Guiness Beer Can

XK
1

The centrifugal advance springs in my XK distributor are
worn, allowing centrifugal advance to come in much too soon.
Rather than remove/disassemble/reassemble/reinstall the
distributor numerous times while trying various replacement
springs and determining the advance curves with a timing
light on a running engine, a simple advance curve tester can
be made in a few evenings.

It basically comprises a wooden base about 1’ by 2’, having
a variable speed electric drill positioned so its chuck is
pointing up.

In the chuck is mounted a horizontal circular plywood degree
wheel. The degree wheel has an axle in its centre made from
threaded rod, washers and lock nuts. The drill spins the
degree wheel at various rpms. On the top surface of the
degree wheel is a set of degree markings, made by
photocopying a protractor onto white paper.

A Lucas distributor is held by a bracket, so it is
positioned vertically, directly above the centre of the
degree wheel. The drive dog on the bottom of the distributor
shaft engages a wooden piece screwed to the top of the
degree wheel at the centre of the degree wheel. The wooden
piece has a recess carved out to accept the drive dog.

The drill spins the degree wheel which spins the distributor
shaft

A black fixed pointer is arranged to point at the degree
markings on the top of the degree wheel.

An ignition coil and 12v battery are wired up to the dizzy,
forming a primary ignition circuit. Ground the dizzy body
to the battery with a wire and alligator clip. I wired the
dizzy as negative ground, to allow a modern electric
tachometer to be wired in, which indicates engine rpm

Hook up 6 spark plugs by grounding each ground electrode to
the 12v battery, and providing some plug wires leading from
the distributor cap to each plug
Alternately, a single spark plug can be used, receiving its
high voltage directly from the coil; then the distributor
cap, rotor and wires need not be used or even installed

A timing light is wired in, and positioned to shine its
flashing light beam at the degree wheel and fixed pointer.
If you have a setback timing light, most of the degree
markings on the degree wheel are not needed.

Then, to determine centrifugal advance curve, just rotate
the degree wheel slowly ccw by hand until the timing light
flashes. Position the pointer so it points at 0 degrees on
the degree wheel. Then run the drill slowly and the timing
light will flash as the 0 degree mark passes by the pointer.
To determine the centrifugal advance curve, just run the
drill at various speeds, and record the amount of indicated
advance as a function of engine rpm.

To determine the vacuum advance curve, run the drill at a
slow constant speed, and apply vacuum to the vacuum advance
fitting. Record amount of vacuum advance as a function of
vacuum applied

The black pointer is made out of the Guiness can.

http://www.jag-lovers.org/snaps/snap_view.php3?id=1140898172
http://www.jag-lovers.org/snaps/snap_view.php3?id=1140898937--
Gary Grant S818919DN
Ottawa, Canada
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–

2

In reply to a message from Gary Grant sent Sat 25 Feb 2006:

The degree wheel has degree markings formed by phoptcopying
a protractor, then enlarging and printing out the image to
match the diameter of the wooden disc, then the paper image
is glued on the top of the wooden disc.

To allow the degree wheel and timing light to indicate
distributor vacuum and/or centrifugal advance in degrees of
crankshaft rotation, instead of degrees of distributor shaft
rotation, I blanked out the photocopied real markings and
substituted new markings showing twice what the real
markings had been.
In other words, the ‘real’ 10 degree marking has been
replaced by a 20 deg marking. The ‘real’ 20 deg mark is now
a 40 deg mark.

I believe this compensates for the fact that the distributor
shaft (and degree wheel) rotate (when installed in an
engine) at 1/2 the engine rpm. Two revolutions of the
crankshaft (720 deg) take place when the distributor shaft
rotates once(360 deg).

The electric tach already conveniently indicates engine rpm,
not distributor shaft rpm.

I believe a setback timing light (if used with only a single
mark on the degree wheel) would indicate advance as a
function of distributor degrees of rotation, rather than
degrees of crankshaft rotation.–
Gary Grant S818919DN
Ottawa, Canada
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
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3

Gary,

I think Rube Goldberg is alive and living in your body. :wink: What an
ingenious piece of equipment. Now the question is, do we have the original
specs for the advance curves as a basis for comparison?

How much will this contraption disassemble? Once you’ve played with your
carbs and have them right, I wonder if you might consider circulating it
(sans drill and vacuum pump) like the movie that’s somewhere out there.

“Mark 1” Mark Stephenson
'52 XK120 S673129, '59 Mk1, '84,'85,'86 &'95 XJ6-----Original Message-----
From: owner-xk@jag-lovers.org [mailto:owner-xk@jag-lovers.org] On Behalf Of
Gary Grant
Sent: Saturday, February 25, 2006 1:37 PM
To: xk@jag-lovers.org
Subject: [xk] Distributor Advance Curce Tester Made from Guiness Beer Can

The centrifugal advance springs in my XK distributor are
worn, allowing centrifugal advance to come in much too soon.
Rather than remove/disassemble/reassemble/reinstall the
distributor numerous times while trying various replacement
springs and determining the advance curves with a timing
light on a running engine, a simple advance curve tester can
be made in a few evenings.

4

In reply to a message from Mark Stephenson sent Sun 26 Feb 2006:

Hi Mark,

I hadnt really thought about being able to ship the tester
around to anyone, as it doesnt really disassemble very
easily, and would occupy a volume of at least a few cubic
feet. The drill would have to go with it, as the wooden
cradle holding the drill in place may not exactly fit the
shape of anyone elses drills. I suspect that if it were
dropped on its side during shipping, the arm holding up the
distributor would probably twist out of alignment with the
drill axis. That alignment between drill axis and
distributor shaft axis is the only important alignment in
the whole rig…when these are not aligned, the degree wheel
starts shaking and vibrating a lot at 2000 rpm.

I am having fun playing with it. Its fun to run the
distributor with cap and wires and 6 spark plugs hooked up.
Then it is fun to watch the plugs firing away in sequence,
and to listen to the high voltage sparks crackling and
jumping between the plug electrodes.

It is perhaps even more fun to omit the cap, wires and
rotor, and have the coil fire just one sparkplug directly.
Then it is fun to watch the points opening and closing. I
think this particular capacitor may be going bad as there
seems to be more sparking between the points than I would
have expected to see.
Every once in a while, a small blue spark can be seen
jumping from the moveable point contact down to the points
plate, which is sort of unexpected. Perhaps a thin layer of
oil or grease building up on the edge of the points and on
the breaker plate is allowing this to happen.

Its also fun to watch the vacuum advance pull the points
plate clockwise, advancing the timing. The coil spring
inside the vernier mechanism then pushes the points plate
counterclockwise, back to its resting position, when vacuum
is released. At first this mechanism was sticking in the
advanced position until I cleaned it up a little. The
vernier oil wicks are broken off, causing lack of
lubrication and sticking of the vernier assembly.

Previously, I thought I had found somewhere in the Archives,
and printed out, some specs on the various Lucas distributor
centrifugal advance curves. Im going to go and try to find
those specs now.
What I dont think I have seen, however, is at what level (or
levels) of vacuum, is the vacuum advance supposed to be
measured? And what amount of vacuum advance is there
supposed to be?

On this particular spare distributor (DVX6A 40276A GC53-951,
with 5-17-10 and 419307 stamped on the hex portion of the
vacuum advance can), believed to be from a MkVII, Im
measuring 15 crankshaft degrees of vacuum advance, when
applying 25’’ Hg vacuum. Seems a little excessive, but
perhaps it may be within spec.–
The original message included these comments:

specs for the advance curves as a basis for comparison?

–
Gary Grant S818919DN
Ottawa, Canada
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
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5

OK, then how about plans? :wink:

Now that you bring it up, I’m kind of interested in how the mechanical and
vacuum advance interact. Are they purely additive or does the advance of one
affect how much the other advances.

I agree it is fascinating, before they used computer circuitry for almost
everything, the mechanical means used achieve a similar result. For example,
I find the way that various turn signal mechanisms work, extremely
interesting, and how they solved the problem of self-canceling. My 120 is a
marvel of simplicity and completely different from my Mark 1.

Mark-----Original Message-----
From: owner-xk@jag-lovers.org [mailto:owner-xk@jag-lovers.org] On Behalf Of
Gary Grant
Sent: Sunday, February 26, 2006 9:19 AM
To: xk@jag-lovers.org
Subject: RE: [xk] Distributor Advance Curce Tester Made from Guiness Beer
Can

In reply to a message from Mark Stephenson sent Sun 26 Feb 2006:

Hi Mark,

I hadnt really thought about being able to ship the tester
around to anyone, as it doesnt really disassemble very
easily, and would occupy a volume of at least a few cubic
feet. The drill would have to go with it, as the wooden
cradle holding the drill in place may not exactly fit the
shape of anyone elses drills. I suspect that if it were
dropped on its side during shipping, the arm holding up the
distributor would probably twist out of alignment with the
drill axis. That alignment between drill axis and
distributor shaft axis is the only important alignment in
the whole rig…when these are not aligned, the degree wheel
starts shaking and vibrating a lot at 2000 rpm.

6

Mark,
Assuming that the Jags work the same way as engines I have worked with in
the past, The vacuum advance advances the timing based on manifold vacuum.
Most of them were ported to the manifold vacuum so that the vacuum port was
on the filter side of the throttle butterfly valve when at idle. An idling
engine does not want much advance, so therefore no vacuum signal to the
vacuum advance diaphragm. It is like when you want to start a Model T, or
other cars of similar vintage, you push the spark advance lever all the way
to “SLOW” when you pull on the crank. This saves you from a broken arm!
Under high load, wide open throttle, the manifold vacuum will be low,
hence, very little vacuum advance. At cruise, part throttle and lower load
on the engine, the manifold vacuum is higher, and because the engine can
tolerate more advance under light load, the vacuum advance gives the engine
what it wants and saves you gas at the same time, while keeping the
temperatures down. The centrifugal advance is just that; it advances the
timing as speed increases. Usually this happens in two stages. That is
accomplished with two springs of different spring rates. One weaker spring
works to oppose the centrifugal force of the weights up to a certain speed
and gives the advance v.s. r.p.m. curve a certain slope. Then the other
spring takes over, and because it has a higher rate, the increase in
advance is much slower from that point on.
Roar

[Original Message]
From: Mark Stephenson mark@jag-lovers.org
To: xk@jag-lovers.org
Date: 26.02.2006 14:56:57
Subject: RE: [xk] Distributor Advance Curce Tester Made from Guiness Beer
Can

OK, then how about plans? :wink:

Now that you bring it up, I’m kind of interested in how the mechanical and
vacuum advance interact. Are they purely additive or does the advance of
one
affect how much the other advances.

I agree it is fascinating, before they used computer circuitry for almost
everything, the mechanical means used achieve a similar result. For
example,
I find the way that various turn signal mechanisms work, extremely
interesting, and how they solved the problem of self-canceling. My 120 is
a
marvel of simplicity and completely different from my Mark 1.

Mark

-----Original Message-----
From: owner-xk@jag-lovers.org [mailto:owner-xk@jag-lovers.org] On Behalf
Of> Gary Grant
Sent: Sunday, February 26, 2006 9:19 AM
To: xk@jag-lovers.org
Subject: RE: [xk] Distributor Advance Curce Tester Made from Guiness Beer
Can

In reply to a message from Mark Stephenson sent Sun 26 Feb 2006:

Hi Mark,

I hadnt really thought about being able to ship the tester
around to anyone, as it doesnt really disassemble very
easily, and would occupy a volume of at least a few cubic
feet. The drill would have to go with it, as the wooden
cradle holding the drill in place may not exactly fit the
shape of anyone elses drills. I suspect that if it were
dropped on its side during shipping, the arm holding up the
distributor would probably twist out of alignment with the
drill axis. That alignment between drill axis and
distributor shaft axis is the only important alignment in
the whole rig…when these are not aligned, the degree wheel
starts shaking and vibrating a lot at 2000 rpm.

7

In reply to a message from Gary Grant sent Sat 25 Feb 2006:

Gary, I wish I had your ability! But for those of us who struggle
to accomplish much simpler repairs, are there any companies with
distributor machines that can properly rebuild and set-up a Lucas
distributor? If so, what would that cost?

Mike Balch–
The original message included these comments:

light on a running engine, a simple advance curve tester can
be made in a few evenings.

–
'53 OTS
Des Moines, Iowa, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
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