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sidevalve5Hi Dave,
Many thanks for the post. Just to clear up one point about ignition systems on small engines and tractors. They all work along the same principle with a current flowing through the primary windings of the coil generating a magnetic field. Which in turn saturate the secondary windings with lines of magnetic force. When the current flowing through the primary is interrupted, the magnetic field rapidly collapses, which cuts through the secondary, generating a very high voltage which seeks a path to ground. This arcs through the spark plug electrode gap, producing the spark that ignites the fuel/air mixture.
Obviously the higher the low tension voltage in the primary, the greater high tension one is generated in the secondary, thus a bigger spark. With a points/condenser system, just as the points open, the current that was flowing through them seeks a path to ground. The primary coil has a degree of resistance which prevents this. During the opening sequence, the points have a tiny gap and the current can arc and go to ground through that, its an easier path than through the primary windings. The purpose of the condenser (a capacitor in all but name) is to take the current away from the points as they open and store the electrons until the points close again when the current will then flow though the primary. The capacitor has two functions, to prevent arcing at the points and to store electrons so they can be used to boost the current when the points close. More current though the primary and the speed at which the current is stopped from flowing through the primary increases the rapidity of the collapse of the magnetic field which generates a higher voltage in the secondary. The condenser is a vital part of the points system and one that is often over looked when a service or overhaul is carried out. I know this to my cost, wish I had studied ignition systems in more detail when I was younger. I got to the level of understanding the principles, but not how the different components operated in unison and the actions of each upon the others. Could have saved hours of trying to start motorcycles, garden tractors and mowers. Have found from experience and research small engines need a 6mm air gap spark to operate well. Under cylinder compression conditions, it is harder for the HT current to pass a gap. So for it to pass say a 0.025” gap consistently and with sufficient intensity under pressure, a 6mm air spark gap is a reasonable equivalent. A spark from a plug resting on ground is no test at all.
In the Kettering system the primary is supplied with a permanent low tension current from a battery. With magnetos, the current is supplied when the magnet passes close to coil of copper wire wrapped around a soft iron core. Lines of magnetic force that go in one direction are produced in the iron core as the north end of the magnet passes over it. These lines change direction when the south end passes over. The point at which the lines of force change direction is called flux reversal and in that instant a voltage spike occurs in the primary and it is when the points open. In the transistorized module you have mentioned, there is a sensor that ‘sees’ the flux reversal, the sensor operates the transistor, which is in simple terms is a switch, just like the points are. Current is stopped from flowing to the primary. The big advantage of the transistorized system over the points/condenser type is the time it takes to stop the current flowing through the primary is almost instant and complete. The collapse in the magnetic field is more rapid, the voltage generated in the secondary higher. With points there is some milliseconds where the current is going to ground through the contact surfaces just as they part, even with a good capacitor. The collapse in the magnetic field is not as rapid.
Have had an 8mm air gap spark from a transistorized module and modern engines are generally easier starters than the old stuff us silly buggers mess about with. But have found as long as a spark of 6mm is obtained with the lower state of tune of an industrial engine, it is perfectly adequate.
To stop interference to TV’s there is a resistor in most plug caps. If you had a very weak spark, you may use a non resistor type. But have found if I achieve a 6mm spark, the resistor type is OK.
If you still want to do a Kettering system, think you will need a two wire 6v ignition coil. In the UK they are available on ebay as cheap as chips, presume its the same in France. Personally if the replacement flywheel is the same as the original, I would still get the hub off and take it from there. Would not be keen on using the machine without fan cooling and the affect of the flywheel inertia. If the coil tested good, would fit a Meco unit. Maybe if the coil was duff, I could contemplate a total loss battery system. The faff of the limitations of the battery would put me off. Much prefer to get it out of the shed, check the fuel, turn it on, choke, tickle, a pull on the rope and off she goes.
Phew, hope I have explained everything in terms you may understand. Expect an electronic engineer would do a better job than me at it. Off now to continue working on a nearly new MacAllister strimmer that is very hard to start because it has a 4mm spark. It is a piece of Chinese plastic crap, but have managed to get the coil off. Got to see if I can get an after-market replacement. Am a big fan of transistorized ignition, but have found a few that underperform. Am not sure if the vibration and heat generated by a small engine damages the circuit components over time. I have no way of testing them, so its just a replacement job.
Grahame
