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Spark plugs, magnetos and oh my aching leg!

Magneto ignition - perhaps the bane of antique motorcyclists.  If any component of the motorcycle can elicit disdain, fear and loathing, it is the poor old under appreciated magneto.  Just the thought of having to work on one runs shivers up many a veteran rider's spine.  Why does the magneto have such an aura of the supernatural about it?  Probably because most folks don't understand how they work.  It's not their fault, even back in the heyday of the motorcycle magnetos, only a few skilled craftsman were trained in their operation.  Fast forward 70 years and there are less than a handful who really understand them and are willing to undertake repairs.  So it is not at all surprising that most riders are a little wary of the mysterious magneto. It's well beyond the scope of this article to delve into the depths of the many magneto designs and theory.  But we will look at some of the causes of difficult starting with magneto ignition.  We'll assume that you've already eliminated other possible problem areas such as fuel delivery and carburetion, timing, rings, valves etc.

That's me checking the internal timing on a Splitdorf NS magneto

Spark Plugs
While this is not a magneto problem, they're breakdown symptoms can easily be confused with that of the magneto.  The spark plug is at the end of a rather long chain of events.  What starts out as a simple magnetic field in the magneto is transformed and amplified into an electrical discharge at the spark plug of over 20,000 volts.  The correct spark plug and gap is crucial to proper ignition.  Spark discharge is a breakdown in the air insulator between the electrode and ground.  Yes - air is an insulator, a pretty good one at that.  As voltage builds at the electrode, atoms in the air within the air gap gain electrons.  This process is called ionization.  Ionization turns the air gap into a low resistance path between conductors.  When a spark plug fires under compression, the quality of the spark will be very different from when it fires under ambient pressure.  Barometric (air) pressure at sea level is approximately 30 lbs. per square inch.  If cylinder compression is 75 psi (measured with a compression gauge), this is actually 75 psi greater than ambient pressure because the compression gauge is zeroed at ambient pressure.  It is much more difficult for a spark to jump the gap under higher pressure, because higher pressure makes the air a better insulator.  A plug may throw a strong spark when held against the side of a cylinder for testing, but when installed in the cylinder and subjected to an additional 75 lbs. of pressure it may not.  This can be indicative of either a weak magneto or deteriorated plug wires.  It can also be caused by a spark plug's insulation breaking down.  Electricity will always seek the path of least resistance to ground.  If the increased resistance at the air gap caused by high air pressure is greater than the plug's insulation resistance, the plug will short to ground and not spark.

Using a  spark plug tester - a great way to check your plugs
How many times have you held a spark plug against a cylinder head and kicked over the engine to see if you had a spark?  Did you have good spark on the cylinder head, but when the plug was put back in the head the $#^@ machine still wouldn't fire?  Earlier we said that compression has a significant effect on the spark plug.  This brings us to the question of how to tell if the plug is working properly inside the cylinder.  In the old days many spark plugs were rebuildable.  Champion and others companies developed Spark plug testers to determine if the plug was defective.  This tool can be very helpful in identifying ignition problems today.  It consists of an air-tight chamber with three threaded spark plug holes.  There is a viewing window and a polished, mirror like surface to allow easier viewing of the spark.  An air line with pressure regulator is connected to the chamber.  Spark is provided by an internal Ford Model T coil which gets 6 volts via a 120 to 6 volts step down transformer.
    If you've tried the spark plug against the head test and had a good spark and all other systems (fuel, timing, rings and valves etc.) seem to be OK, this device may show where the problem lies.  A few years ago I was searching for a set of original, period Splitdorf Green Jacket spark plugs for my 1916 Indian Powerplus.  I went down to visit a spark plug collector and spent a very enjoyable afternoon searching through and testing many plugs.  After locating a half-dozen Green Jacket candidates we broke out his dusty old spark plug tester.  Four of the six sparked OK under ambient pressure, but exhibited little or no spark under 60 lbs. of pressure.  The other two would not spark at all. Since these are rebuildable plugs we first tried taking them apart for a thorough cleaning.  This helped some, but not entirely.  After trying various combinations of cores and bodies, we were getting nowhere fast.  Seeing the look of desperation in my eyes, he said "wait here".  A few minutes later he emerged with two N.O.S Splitdorf cores.  We put them into the best bodies and tried them out.  The result was a nice hot, blue spark at ambient pressure and at 60 psi.  I was in business.
    Without a spark plug tester I would have assumed the plugs that sparked at ambient pressure (laying plug against the head type test) were good.  No doubt I would have kicked and kicked that motor until my leg throbbed and I was blue in the face without ever getting it to fire.  I'd have continued to believe the plugs were OK and probably gone off troubleshooting in a different and unnecessary direction.
    These old testers can occasionally be found at antique auto swap meets or advertised in publications such as Hemmings Motor News.  I was fortunate in finding mine at the AMCA national meet in Wauseon Ohio about 4 or 5 years ago.  It worked good until recently when the coil finally gave up.  Fortunately there are a few craftsmen in the Model T hobby who restore these coils.  I sent it off with instructions to do whatever it took to put it in working order.  In less than two weeks I had the coil back, restored and adjusted for under $40.00.  My spark plug tester was back in operation.  Whenever I'm having trouble starting a machine, and I know that the fuel system is working OK, I'll pull the plugs and wring them out on the Spark plug tester.  Then I know for sure if there is a spark plug problem or not.
    A spark plug tester can usually be had at a pretty reasonable price.  If you see one at a swap meet, consider adding it to your shop tool collection.  It could more than pay for itself in the time and aggravation saved troubleshooting a cantankerous old motorcycle, not to mention a week's worth of Ben-gay for your sore leg.

    The purpose of the condenser is to ensure a clean, non-sparking, opening of the points.  Without a condenser electricity would jump the point gap, quickly destroying the contact points.  Condensers deteriorate with age - period.  Usually when a condenser fails it becomes open, not shorted.  An easy way to tell if the condenser is bad is to watch the points when the engine is idling (if it will operate with the point cover removed).  If you see a large yellow spark when the points open instead of a small blue spark, there is a good chance the condenser is open or defective.  Early condensers were made of laminated layers of foil, mica and paper.  While these were of excellent quality when new, in time the laminations may separate or become moisture saturated which causes them to deteriorate.  Modern automotive condensers or capacitors can be used to replace the old laminated type.  Just be sure to get one that has a similar voltage and microfarad rating to the original.  Generally speaking, very early magneto condensers were .07 - .15 microfarads, later condensers were .2 -.35 microfarads.  I've found that modern 630 volt, .2 microfarad capacitors available from electronic supply houses to be about right for many magnetos.

Here is a modern condenser ready to install in a Splitdorf NS mag

    There are two main problem areas with coils: insulation break down and vibration damage.  Insulation breaks down overtime and allows high voltage to short to ground.  Shellac or varnish was commonly used as an insulators and sealer.  On a hot day, the shellac could melt and actually oooze out of the coil.  The other problem is vibration.  A coil is made up of a primary and a secondary winding.  The primary is a few turns of heavy gauge wire, the secondary is made up of many turns of very fine wire - often as fine as a human hair.  Through rough handling or extensive vibration these windings can break or short.  Today, coils can be rewound with modern wire and an epoxy type sealer that is virtually impervious to heat and deterioration.

An original coil next to a freshly rewound one.  Which would you rather rely on?

Magnetic field strength
The majority of magnetos used on antique motorcycles have a horseshoe shaped, permanent magnet.  This magnet is usually made of a hard steel alloy.  These are different from soft iron used magnets used in coils or generator pole shoes.  The magneto's magnet is permanently charged at the factory and should remain charged for many years.  However, over time the strength of it's magnetic field can deteriorate.  Several things can cause this; If the mag has ever been disassembled and a "keeper" was not used on the magnet to maintain the circuit, the magnetism will have been significantly reduced.  Shock and vibration can cause a loss of magnetism as well.  Poor compression can weaken the magnet (more on this later). If nothing else, age takes its toll as well.  The bottom line is that your magnet may need to be recharged.  This takes a special recharging machine (actually a big electro-magnet) designed specifically for this purpose.  It's amazing how a freshly recharged magnet can improve a magneto's performance.

George Yarocki recharging a Splitdorf NS mag

Venting the magneto
    We talked about ionization of the spark plug air gap - that's a good thing.  Unfortunately, ionization can occur internally to the magneto - that's a bad thing.  Virtually all magnetos have what's called a Safety Gap.  The purpose of this is to provide a path to ground for the spark in case the high voltage leads are not connected to a plug or other ground.  As the magneto spins and develops high voltage, there must be some place for it to go otherwise damage to the magneto's insulation and other parts will result.  The safety gap is designed and adjusted for a spark to jump only when the potential is well above the expected level at the spark plug.  This works essentially the same way as a safety valve on a boiler does.  Ionization can occur at the safety gap and at the distributor block.  This is where venting is important.  Some magnetos are made with an air vent, others rely on non-air tight joints for ventilation.  On old motorcycles, magnetos are often subjected to moisture; either from rain, humidity or an occasional washing.  Some people try to seal the magneto at its joints with silicone sealer or similar products.  This may help prevent moisture intrusion, but it also causes ionized gas to be retained within the magneto.  A buildup of ionized, conductive gas can cause internal shorting.  That means if there is shorting in the magneto, there is reduced or no voltage getting to the spark plug.  This may not be a problem when first starting, but may become evident after running for a while. Never seal your magneto air tight. Let it breath.

Kicker ratio
    Magneto ignition requires a motor to turn over faster than battery and coil systems in order to generate a good spark.  Manufacturers carefully design the kick starter gear ratio to be able to spin the engine over at the required speed.  Some machines were available with either type of ignition.  Over the years kicker parts may have been replaced and mismatched. Check to ensure the kicker and gear ratio are correct for your ignition system.

Starting procedure
    Every machine has its own starting drill.  As time goes on things such as timing, compression and spark plugs deteriorate.  The starting drill that worked two years ago may not work now.  Generally speaking, when starting a machine with battery and coil ignition, the engine is turned over slowly until it’s coming up on the compression stroke.  Then it's ready to kick into life.  With a magneto this may or may not work.  It may be necessary to get it set a half or quarter stroke before coming up on the compressions stroke.  This will allow the rotating parts of the engine to gain speed and inertia before the piston encounter compression which tries to slow down the rotating mass.  The extra rotating speed attained before compression begins will generate a better spark at the plugs.

Platinum vs. Tungsten points
    Over the many years a magneto has been on the road, it is likely that some service has been performed on it.  The most likely being replacement of the points and oiling the bearings.  Most magneto manufactures specified the use of platinum points as opposed to tungsten points commonly used in battery and coil ignition systems.  If tungsten points have been installed in your magneto, it may work fine for a while but it will soon lose its ability to provide a hot spark under low RPM.
Platinum is usually alloyed with iridium (80% platinum/20% iridium) to attain toughness and hardness of the contact point, which gradually enhances its electric value.  Pure platinum would hammer under the action of the contact breaker.  Platinum has the property of a very high fusing point and does not oxidize under ordinary conditions.  It has a very low surface resistance, which remains throughout the life of the point.  While it is subject to a certain  building up and pitting process, this process extends over the entire surface of the point, with the result that the contact area is not impaired.  It has long life and can be used down to the last few thousandths of an inch.
    Tungsten points are primarily used with battery and coil ignition.  Tungsten is extremely hard. It withstands the hammer type blows caused by the high spring tension, which is necessary in high speed interrupters and to maintain good contact at the point surfaces.  It is not rapidly consumed by electrolysis, but it is subject to oxidation, and inherently has a high resistance.  This oxidation results in a high resistance oxide film coating on the points, reducing the flow of current.  Thus a high voltage across the points is necessary (usually 6 volts from the battery).
    The disadvantage of tungsten points in magnetos is the oxidation of the points and the high surface resistance. Both of these factors prevent the magneto primary current building up to its maximum value, resulting in weak current and hard starting.  Arcing across the points is more common with tungsten than with platinum, and it is more difficult to determine when a condenser is defective.  When platinum points are used, extreme arcing is always an indication of a defective condenser.  Tungsten points require a greater condenser capacity to overcome arcing than do platinum points.
So how do you tell if your points are platinum or tungsten? First off platinum is a much softer than tungsten. A fine tooth jeweler's file will cut them very easily.  Tungsten contacts are much harder.  Try your file on an old set of tungsten points from Aunt Betsy's 69 Buick.  Then carefully dress your magneto's points.  If they are platinum, you'll be able to tell a big difference.  Another way the old timers used to tell the difference is to put a few drops of nitric acid on the contact.  If the acid eats into then points they're not platinum.

How poor compression weakens a magneto
    If the engine has good compression the piston should stop on a quarter stroke, not dead center.  In the magneto, this results in a free path for magnetic lines-of-force forming virtually a complete magnetic circuit.  If the engine has poor compression, even in one cylinder, the piston may stop on dead center.  In the magneto, this causes the path for the magnetic lines from one pole-piece to the other to be very poor.  Without a good magnetic circuit, the residual magnetism will deteriorate.  You may have heard never to remove a magneto's magnet without placing a "keeper" across the bottom.  This keeper maintains the magnetic circuit the same way.


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