Ignition Matters PDF Print E-mail
Written by BMWVMCA NEWS   
Tuesday, 05 August 2008

Ignition Matters

A coil either works or it doesn't. A weak magnet will yield a weak spark. Too close points gap will also yield a weak spark. A leaking condenser will yield a yellowish weak spark.

One of the reasons for coil failure is caused by heat expansion. Because the winding in a magneto type coil needs to be very fine, the wires break easily, and of course coils warm up and cool down frequently during their lifetimes. This frequent expansion and contraction eventually causes the fine wires in the winding to fracture. When the coil is cold, the broken wire ends touch, and the coil works. When the coil heats up and expands, the wires begin to separate. There is often enough voltage to cause the electrical impulse to leap across the gap and continue running.

When the engine is shut down, the weakened coil no longer has any cooling air flowing over it, and it equalizes with the ambient engine temperature, actually becoming warmer than under operating circumstances. The wire gaps increase with the rise in temperature.

It’s hard to believe that an engine that ran so strong a few minutes ago, and we just shut down so we could stop for a few minutes will not restart. Frustrating?

We can either kick away and lose some fat, or go for a walk. When the engine cools down, the gaps in the winding close, and once again we have spark.

We will not always be so lucky, because once a coil has fractured wires in its winding, it will fail more often, until it no longer works at all.

Bench Mark Works has replacement coils. The first batch they had made were troublesome. Vech tells me he had the manufacturer install a ground strap, which seems to have cured their earlier problems.

It is always a good idea to carry a spare coil along with an extra set of plugs, a condenser and light bulbs in your tool kit.

These magneto coils are actually  dual units. Imagine a square iron bar the full length of the coil, with a primary winding wrapped around it for most of its length.

When a small electro magnetic force (EMF) is passed through the iron bar inside the primary winding, which is also known as a low tension coil, a magnetic field is created each time one of the two poles in the magnetic rotor passes near the iron core. At the point where the EMF is greatest, called Abrisz, is where we want the points to open, breaking the EMF.  We need to have a condenser, really a capacitor attached in parallel with the breaker points. The capacitor works like a storm cloud, storing and releasing energy.

The following is an explanation from Selkirk Selair Aircraft Maintenance School:

“The breaker points are timed to the rotating magnet by means of a cam, which allows them to remain closed until the precise moment they can be opened to interrupt the current going through the primary windings.  Because electricity is flowing through the points when they open, arcing will occur, which will damage the points in short order if not prevented.  To do this, a capacitor is placed in parallel with the points.  Since the capacitor has an initially higher resistance than the low-resistance points, no electricity flows into it.  However, when the points begin opening and resistance between the points increase, the electricity selects the easier path through the capacitor.  Just as the capacitor is filled, the electrical movement into it stops, but the points are now too far apart for arcing to occur.”

Our low tension coil still does not produce enough voltage to jump across the spark plug gaps. For this, we need a transformer.

Next, let us wind double strands of very fine wire around the primary wires of the low tension coil. We’ll call these strands the ‘secondary winding’. At one end of these fine strands, we’ll attach our spark plug leads. At the other end, we’ll attach them to ground.

When a magnetic impulse or EMF is created in the low tension coil, or primary winding, it passes through the fine wires of the secondary coil as a magnetic field. The more fine wires we can wrap close in to the primary windings, the higher the voltage output in the secondary winding, and because we can’t get something for nothing, the lower the amperage. Of course, that’s what we need, high voltage and very low amperage.

Remember we used double strands? That is how we get a dual coil.

One of the pitfalls of using a magneto is that when the magnets spin slowly such as start up, the spark will be weak. Of course that leads to a magneto’s strength: when the magnets spin fast, such as at higher RPM’s, the spark is strong where it is needed.

In addition, weak magnets mean a weak spark, and conversely strong magnets mean a strong spark.

We can’t do much about kicking our engines to life any faster, but we can improve the strength of the magnets.

Randy Franks has done some research into recharging the type magneto  in our 1951 through 1969 twins. Randy has them charged while the rotor is in the magneto body where the magnetic flux can be contained by the pole shoes of the magneto body. Randy suggests that the recharged magnetos be installed as a unit, and the rotor not be separated at any time, lest some of the magnetic flux bleed off .

Randy currently has several recharged magnetos for sale. He advised me that he is not offering a recharge service at this time. He does not have any R68 or R69 magnetos.

You may contact him at:
818 360 4008

In the next issue, we will discuss Battery/Coil ignition, sometimes known as the Kettering System.

Below is a photo of how Todd Rasmussen modified his R51/2 ignition system. The original used a rotor and distributor cap, much the same as a pre-electronic ignition system in a car.
Todd eliminated the rotor and cap in favor of a Dyna dual coil. Both spark plugs fire at the same time, leaving the cylinder on its exhaust stroke with a lost spark. The dual coil system reduces problems by reducing two of the components most likely to fail.  More on this in the next issue of the VMCA News.

Last Updated ( Monday, 08 June 2009 )
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