LUCAS MAGNETOS-
THE MOST CURSED AND LEAST UNDERSTOOD
PART OF A BRITISH MOTORCYCLE
The primary advantage of a
magneto is that it works independently of a battery. In fact you don't really
don't even need a battery. Magnetos act as a generator and create their own low
or primary voltage, then act as a spark coil and transform the low voltage into
high voltage (around 20,000 volts). The faster they spin, the more voltage they
put out.
Almost everybody has probably heard of the "reverse
Polish logic" used in early LED calculators. I think Lucas engineers used
reverse British logic in the design of the mag as it's designed backwards from
conventional wisdom. Most mags, aircraft and tractor, have a stationary coil and
a rotating magnet. Lucas, on the other hand, have a rotating coil and a
stationery magnet (except for limited production SR series). The main
advantage to this design is a smaller and more compact package.
Around
fifteen years ago I got serious about working on Lucas magnetos out of necessity.
After finishing the restoration on my Vincent, I found that I couldn't get the
darn thing started. It would fire on one cylinder but not on the other. I would
take out both plugs, kick the engine over, and both plugs would spark (more on
this later). After doing the Phil Irving aerobic work out for several days I
finally convinced myself of the obvious. There must be something wrong with the
mag.
Vincent
mags are a special problem due to the V-twin configuration. Magnetic fields have
a north pole and a south pole, and they're 180 degrees apart. This works great
for vertical twins such as Triumph because the magneto is supposed to fire every
180 degrees (or 360 degrees of crankshaft rotation). It is important to note
that the points should open just as the magnetism induced in the armature is
changing polarity and the magnetic field is collapsing. But this situation is
complicated as the Vincent is a V-twin and only one plug (rear cyl.) gets an
optimum spark. Because the second plug fires 25 degrees after the optimum point
of field collapse the other (front cyl.) gets a mediocre spark. My mag had
degraded to the point that one plug was getting a weak spark, and the other was
getting a VERY weak spark.
Among
other things, there are three main sources of problems with every Lucas mag in
existence today-
(1) Loss of magnetism. The
primary enemies of a permanent magnet are heat and shock (from vibes) and of
course, age. I've tested the primary AC voltage in mags before and after
magnetizing and have found an increase of around 10% to 20%. This means a
comparable increase in secondary voltage. Not surprising considering most mags
are around 35-50 years old.
(2) Leaky capacitor. In the
case of a magneto the capacitor stores voltage that is created by the collapsing
magnetic field in the armature. In doing so, it also prevents the points from
arcing or burning when they open. In order for a capacitor to do its job, it
must be able to take on an electrical charge and hold it. A leaky capacitor
cannot take a charge because the charge is dissipated as quickly as it
accumulates. I can guarantee every British motorcycle powered by a
magneto is either not delivering maximum spark and/or is on the verge of failing
if the original Lucas capacitor has not been replaced. All capacitors made
in that era were made with (waxed) paper dialectric, and they will degrade with
age.
Lucas (and most other) capacitors made in the
50's & 60's used a paper dielectric. Remember back in the old days when you
did a tune-up on your '57 Chev. (or was it a '65 MGB)? Any time you replaced the
points, you automatically replaced the condenser.
I've tested NOS Lucas capacitors right off the
shelf that are leaky. It's humorous seeing new Lucas capacitors advertised in
catalogs at prices that approach $75 ea. Due to the above mentioned
characteristics, an original Lucas capacitor is the last thing you want
to use. So where is do you get one, Radio Shack? No. Due to the high pulse
currents encountered with ignition circuits, special capacitors must be used.
(For engineers, the dV/dt rating should be >1500 and the DF <.05%)
Capacitors obtained through commercial sources are usually metalized
polyester and will fail.
Eventually all pre-70's Lucas capacitors
become leaky. With time and age, the capacitor goes through a metamorphosis-
first turning into a resistor, and finally into a conductor, i.e., dead short.
That mag that worked great twenty years ago when the bike was parked now emits
only a small spark. I've heard people say their mag will shock the crap out of
you so it must be good. Wrong! House voltage (110 volts) will shock the crap out
of you, and even kill you, but it certainly will NOT spark your spark plug.
But I must urge caution here. The condenser is
located deep within the bowels of the armature, and to replace it is almost a
surgical procedure. Careless handling will do some expensive damage to the slip
ring, especially if the safety gap screws are not removed before the armature is
removed. But that's only half the battle. Even if you did remember to remove the
safety gap screws you will still get another chance to break the slip ring when
you attempt to remove it from the armature. It's made of bakelite (plastic) and
is VERY brittle. It's not unusual for it to become "glued" to the
armature shaft due to corrosion of the brass. And there are usually other
problems that must be attended to such as trueing the slip ring and grounding
surface in a lathe. So, unless you know what you are doing. or have a good stock
of Lucas spares, it is better to trust someone who is more knowledgeable in this
field. You can easily end up spending more by doing it yourself and still not
have a really good mag.
(3) Armature failure- Either by
breakdown of insulation, i.e. high voltage arcing to ground, or open secondary.
The first sign of trouble is difficult starting when hot. The secondary
insulation can be tested with a "megger" to check the insulation
resistance. The armature should be stripped and primary hot and ground wires
disconnected from the capacitor. Ideally it should be above 1000 megohms, and
anything below 20 megohms indicates impending failure. Also, the secondary
winding should be checked for continuity with an ohm meter. A resistance of
around 5K ohms indicates a good winding.
I've seen mags taken off a running motorcycle
that tested "open". (Starts okay when cold, but not when hot.) So why
did the bike run?? The secondary is wound with a very fine wire about the
thickness of a human hair. As a matter of fact, about one mile of it. If an open
circuit develops in the secondary the high voltage can and will jump the gap,
and the bike continues to run. Slowly but surely the ends of the fine wire melt
and the gap gets bigger and bigger. This arcing inside the secondary will also
leave a carbon track to assist the jumping of the spark. While the carbon track
is not a perfect conductor, it's good enough to conduct the high voltage spark
when the mag is cold. After it heats up, the bike misfires. Let it cool
down and it will start okay. Then the cycle repeats itself.
But here's the bottom line. It's really false
economy to use an old armature on a bike you are restoring or are going to ride
any distance from home. The magnet wire used back then was insulated with a
lacquer varnish. Magnet wire used today has a better polyurethane varnish
insulation, and has a higher dielectric strength. Due to the high incidence of
armature failures that I've found, I always recommend rewinding the armature. Or,
don't ride your bike any farther from home than you would want to push it. And
for the above reasons, I would certainly not trust an NOS armature.
Although
highly unlikely, the pick-ups can be another problem area. One chap told me his
bike was cutting out at high speed (5000+ rpm). Yeah, it was a little
temperamental to start but seemed to run okay at cruising speed (3500 rpm). He
was convinced his problem was carburetor so he overhauled them. No luck. He
checked about everything he could think of, but still no luck. He sent me his
mag and I put it on my test stand and ran it up to 1750 rpm. (Which simulates an
engine cruising speed of 3400 rpm.) It seemed to be working okay, or at least
had no obvious problems. After I overhauled and remagnetized it, I put it back
on the test stand again at 1750 rpm. Presto, the problem was now obvious. The
high voltage was arcing through the pickup to ground. Now that the mag was
overhauled it was putting out enough voltage to jump through the pickup to
ground at 1700 rpm. There was no obvious damage to the pickup that could be
detected with the naked eye, but all it takes is a tiny pin hole. It's not
unusual to find leakage points along the mold lines of the plastic pickups even
at low speed.
High compression can be another problem with an
old mag. The plug will fire when removed and the engine kicked over, but won't
fire when put it back into the engine. Why? Because as the pressure is increased,
higher voltage is required to jump the gap. Ergo, a magneto that might work on
an engine with 7:1 compression might not work on an engine with 12:1 compression.
The next step is to reduce the spark plug gap until the engine starts. I've been
through the same scenario myself many years ago. First you reduce the gap to
.015", and the bike starts. Then next year it's necessary to reduce it to
.012" and it runs again. But how long can you keep reducing the plug gap?
Another approach would be to use a plug with a small diameter center electrode
such as Bosch platinum. A small diameter electrode will spark with a lower
voltage, but will not give as hot a spark.
If you're old enough you might remember the
Champion spark plug testing machines at your local filling station. Here is a
good example of the relationship of plug voltage vs. chamber pressure. A spark
plug was placed into the tester and a high voltage wire attached. You increased
the air pressure to the spark chamber to see how much pressure was required to
extinguish the spark. Back then, you didn't just replace your plugs at given
mileage intervals. If a spark plug failed, you identify the ailing plug and
replaced it. Why replace the rest of them if they are still working? Right?
Eventually Champion finally got smart and stopped making plug testers as they
surely affected plug sales.
To have a mag expertly overhauled is not cheap.,
but to replace Lucas parts that you screw up trying to DIY is not cheap either.
If you don't have the special tools or expertise to do it yourself, trust it to
someone who does.
The primary advantage of a magneto is that it works independently of a
battery. In fact you don't really don't even need a battery. Magnetos act as a
generator and create their own low or primary voltage, then act as a spark coil
and transform the low voltage into high voltage (around 20,000 volts). The
faster they spin, the more voltage they put out.
Almost everybody has probably heard of the "reverse
Polish logic" used in early LED calculators. I think Lucas engineers used
reverse British logic in the design of the mag as it's designed backwards from
conventional wisdom. Most mags, aircraft and tractor, have a stationary coil and
a rotating magnet. Lucas, on the other hand, have a rotating coil and a
stationery magnet (except for limited production SR series). The main
advantage to this design is a smaller and more compact package.
Around fifteen years ago I got serious about
working on Lucas magnetos out of necessity. After finishing the restoration on
my Vincent, I found that I couldn't get the darn thing started. It would fire on
one cylinder but not on the other. I would take out both plugs, kick the engine
over, and both plugs would spark (more on this later). After doing the Phil
Irving aerobic work out for several days I finally convinced myself of the
obvious. There must be something wrong with the mag.
Vincent mags are a special problem due to the
V-twin configuration. Magnetic fields have a north pole and a south pole, and
they're 180 degrees apart. This works great for vertical twins such as Triumph
because the magneto is supposed to fire every 180 degrees (or 360 degrees of
crankshaft rotation). It is important to note that the points should open just
as the magnetism induced in the armature is changing polarity and the magnetic
field is collapsing. But this situation is complicated as the Vincent is a
V-twin and only one plug (rear cyl.) gets an optimum spark. Because the second
plug fires 25 degrees after the optimum point of field collapse the other (front
cyl.) gets a mediocre spark. My mag had degraded to the point that one plug was
getting a weak spark, and the other was getting a VERY weak spark.
Among other things, there are three main sources of
problems with every Lucas mag in existence today-
(1) Loss of magnetism. The primary enemies of a permanent magnet are heat
and shock (from vibes) and of course, age. I've tested the primary AC voltage in
mags before and after magnetizing and have found an increase of around 10% to
20%. This means a comparable increase in secondary voltage. Not surprising
considering most mags are around 35-50 years old.
(2) Leaky capacitor. In the case of a magneto the capacitor stores voltage
that is created by the collapsing magnetic field in the armature. In doing so,
it also prevents the points from arcing or burning when they open. In order for
a capacitor to do its job, it must be able to take on an electrical charge and
hold it. A leaky capacitor cannot take a charge because the charge is dissipated
as quickly as it accumulates. I can guarantee every British motorcycle
powered by a magneto is either not delivering maximum spark and/or is on the
verge of failing if the original Lucas capacitor has not been replaced. All
capacitors made in that era were made with (waxed) paper dialectric, and they
will degrade with age.
Lucas (and most other) capacitors made in the
50's & 60's used a paper dielectric. Remember back in the old days when you
did a tune-up on your '57 Chev. (or was it a '65 MGB)? Any time you replaced the
points, you automatically replaced the condenser.
I've tested NOS Lucas capacitors right off the
shelf that are leaky. It's humorous seeing new Lucas capacitors advertised in
catalogs at prices that approach $75 ea. Due to the above mentioned
characteristics, an original Lucas capacitor is the last thing you want
to use. So where is do you get one, Radio Shack? No. Due to the high pulse
currents encountered with ignition circuits, special capacitors must be used.
(For engineers, the dV/dt rating should be >1500 and the DF <.05%)
Capacitors obtained through commercial sources are usually metalized
polyester and will fail.
Eventually all pre-70's Lucas capacitors
become leaky. With time and age, the capacitor goes through a metamorphosis-
first turning into a resistor, and finally into a conductor, i.e., dead short.
That mag that worked great twenty years ago when the bike was parked now emits
only a small spark. I've heard people say their mag will shock the crap out of
you so it must be good. Wrong! House voltage (110 volts) will shock the crap out
of you, and even kill you, but it certainly will NOT spark your spark plug.
But I must urge caution here. The condenser is
located deep within the bowels of the armature, and to replace it is almost a
surgical procedure. Careless handling will do some expensive damage to the slip
ring, especially if the safety gap screws are not removed before the armature is
removed. But that's only half the battle. Even if you did remember to remove the
safety gap screws you will still get another chance to break the slip ring when
you attempt to remove it from the armature. It's made of bakelite (plastic) and
is VERY brittle. It's not unusual for it to become "glued" to the
armature shaft due to corrosion of the brass. And there are usually other
problems that must be attended to such as trueing the slip ring and grounding
surface in a lathe. So, unless you know what you are doing. or have a good stock
of Lucas spares, it is better to trust someone who is more knowledgeable in this
field. You can easily end up spending more by doing it yourself and still not
have a really good mag.
(3) Armature failure- Either by breakdown of insulation, i.e. high voltage
arcing to ground, or open secondary. The first sign of trouble is difficult
starting when hot. The secondary insulation can be tested with a "megger"
to check the insulation resistance. The armature should be stripped and primary
hot and ground wires disconnected from the capacitor. Ideally it should be above
1000 megohms, and anything below 20 megohms indicates impending failure. Also,
the secondary winding should be checked for continuity with an ohm meter. A
resistance of around 5K ohms indicates a good winding.
I've seen mags taken off a running motorcycle
that tested "open". (Starts okay when cold, but not when hot.) So why
did the bike run?? The secondary is wound with a very fine wire about the
thickness of a human hair. As a matter of fact, about one mile of it. If an open
circuit develops in the secondary the high voltage can and will jump the gap,
and the bike continues to run. Slowly but surely the ends of the fine wire melt
and the gap gets bigger and bigger. This arcing inside the secondary will also
leave a carbon track to assist the jumping of the spark. While the carbon track
is not a perfect conductor, it's good enough to conduct the high voltage spark
when the mag is cold. After it heats up, the bike misfires. Let it cool
down and it will start okay. Then the cycle repeats itself.
But here's the bottom line. It's really false
economy to use an old armature on a bike you are restoring or are going to ride
any distance from home. The magnet wire used back then was insulated with a
lacquer varnish. Magnet wire used today has a better polyurethane varnish
insulation, and has a higher dielectric strength. Due to the high incidence of
armature failures that I've found, I always recommend rewinding the armature. Or,
don't ride your bike any farther from home than you would want to push it. And
for the above reasons, I would certainly not trust an NOS armature.
Although highly unlikely, the pick-ups can be
another problem area. One chap told me his bike was cutting out at high speed
(5000+ rpm). Yeah, it was a little temperamental to start but seemed to run okay
at cruising speed (3500 rpm). He was convinced his problem was carburetor so he
overhauled them. No luck. He checked about everything he could think of, but
still no luck. He sent me his mag and I put it on my test stand and ran it up to
1750 rpm. (Which simulates an engine cruising speed of 3400 rpm.) It seemed to
be working okay, or at least had no obvious problems. After I overhauled and
remagnetized it, I put it back on the test stand again at 1750 rpm. Presto, the
problem was now obvious. The high voltage was arcing through the pickup to
ground. Now that the mag was overhauled it was putting out enough voltage to
jump through the pickup to ground at 1700 rpm. There was no obvious damage to
the pickup that could be detected with the naked eye, but all it takes is a tiny
pin hole. It's not unusual to find leakage points along the mold lines of the
plastic pickups even at low speed.
High compression can be another problem with an
old mag. The plug will fire when removed and the engine kicked over, but won't
fire when put it back into the engine. Why? Because as the pressure is increased,
higher voltage is required to jump the gap. Ergo, a magneto that might work on
an engine with 7:1 compression might not work on an engine with 12:1 compression.
The next step is to reduce the spark plug gap until the engine starts. I've been
through the same scenario myself many years ago. First you reduce the gap to
.015", and the bike starts. Then next year it's necessary to reduce it to
.012" and it runs again. But how long can you keep reducing the plug gap?
Another approach would be to use a plug with a small diameter center electrode
such as Bosch platinum. A small diameter electrode will spark with a lower
voltage, but will not give as hot a spark.
If you're old enough you might remember the Champion
spark plug testing machines at your local filling station. Here is a good
example of the relationship of plug voltage vs. chamber pressure. A spark plug
was placed into the tester and a high voltage wire attached. You increased the
air pressure to the spark chamber to see how much pressure was required to
extinguish the spark. Back then, you didn't just replace your plugs at given
mileage intervals. If a spark plug failed, you identify the ailing plug and
replaced it. Why replace the rest of them if they are still working? Right?
Eventually Champion finally got smart and stopped making plug testers as they
surely affected plug sales.
To have a mag expertly overhauled is not cheap.,
but to replace Lucas parts that you screw up trying to DIY is not cheap either.
If you don't have the special tools or expertise to do it yourself, trust it to
someone who does.
