10/04/2005
The Villiers engine, a stalwart of many classic British motorcycles and machinery, often features a distinctive back plate assembly that plays a crucial role in its ignition system. Understanding how this component works, and how to maintain it, is key to keeping these venerable machines running smoothly. This guide will delve into the function of the Villiers back plate, focusing particularly on its interaction with the ignition coil and contact breaker points, and offer practical advice for troubleshooting and repair.
The Villiers Back Plate: A Central Hub
The back plate in a Villiers engine is more than just a mounting surface; it's a meticulously designed component that houses and supports several vital parts of the ignition system. Primarily, it provides a secure mounting for the contact breaker points, the mechanism that interrupts the primary circuit of the ignition coil, and often, the ignition coil itself. The precise positioning of these components is critical for generating a strong spark at the correct time in the engine cycle. The back plate ensures this alignment is maintained, even under the vibrations and stresses of operation.
Ignition Coil: The Spark Generator
The ignition coil is the heart of the spark generation process. In many Villiers setups, the coil is mounted directly to, or integrated within, the back plate assembly. Its function is to transform the low voltage from the magneto or dynamo into the high voltage required to fire the spark plug. This is achieved through electromagnetic induction. The coil consists of two windings: a primary winding with relatively few turns of thicker wire, and a secondary winding with many thousands of turns of very fine wire, wound around a laminated iron core.
Checking or Replacing The Ignition Coil
If your Villiers engine is suffering from a weak or non-existent spark, the ignition coil is a prime suspect. Diagnosing a faulty coil often involves checking its resistance. To do this, it's necessary to disconnect the wire from the coil to the contact breaker points. You will need a voltmeter for this task. The primary winding's resistance should be a mere few Ohms, measured between ground (which can be any point on the iron core) and the terminal where the points wire was connected. The secondary winding's resistance, responsible for the high-voltage output, should typically measure between 3k Ohms and 5k Ohms. This is measured between the terminal that connects to the high-tension lead and ground.
The most common failure mode for an old Villiers ignition coil is internal corrosion, particularly affecting the secondary winding. This winding is made from extremely fine copper wire, making it susceptible to breakage due to vibration or age. If your tests indicate a fault, or if the coil is visibly damaged, replacement is necessary.
Coil Removal and Replacement Steps
Removing the ignition coil from the back plate assembly usually involves a few key steps. The coil is often held in place by "shoe" shaped laminated metal core ends, secured by nuts on the rear of the back plate. To remove these core ends, you'll likely need to undo the nuts on the back. Counteracting the torque from the screws that hold these core ends to the back plate might require the use of a screwdriver. Once the nuts are removed and the core ends are lifted away from the back plate, the coil itself can be carefully pulled free from these shoes.
When sourcing a replacement coil, it's crucial to specify the exact model of your Villiers engine. There are various lengths and end diameters available, and using an incorrect coil will prevent proper fitting and function. Be prepared for replacement coils to be a significant investment; they can cost around £50. Exercise caution when purchasing from online auction sites, as untested or poor-quality items are common.
The Contact Breaker Points
While the back plate is intimately involved with the ignition coil, it also serves as the mounting point for the contact breaker points. These points are essentially a switch that opens and closes the primary circuit of the ignition coil at precisely the right moment. When the points are closed, current flows through the primary winding, building up a magnetic field. When the points open, the magnetic field collapses, inducing a high voltage in the secondary winding, which then jumps the spark plug gap.
Inspecting and Adjusting Contact Breaker Points
With the flywheel removed, you gain excellent access to the contact breaker points. This is the ideal opportunity for a thorough inspection. Look for any signs of pitting or burning on the contact surfaces, which can impede electrical flow and weaken the spark. The gap between the points when they are fully open is also critical. This gap determines the duration for which the primary winding is energized, influencing the strength of the spark. Typically, this gap is set to around 0.015 inches (0.38 mm), though it's always best to consult the specific manual for your Villiers engine model.
Cleaning pitted points with a fine file or emery cloth can sometimes revive them. However, if the pitting is severe or the points are significantly worn, they will need to be replaced. Ensure that the points pivot correctly and that the rubbing block makes good contact with the cam on the crankshaft or magneto shaft that operates them.
Common Problems and Solutions
The Villiers back plate system, while robust, is not immune to common issues:
| Symptom | Potential Cause | Solution |
|---|---|---|
| Weak or no spark | Faulty ignition coil | Test coil resistance, replace if necessary. |
| Weak or no spark | Pitted or incorrectly gapped contact breaker points | Clean, adjust gap, or replace points. |
| Engine misfiring | Loose connections to the coil or points | Check and tighten all electrical connections. |
| Engine misfiring | Worn or damaged condenser (often mounted near the points) | Test condenser or replace. |
| Backfire or poor running | Incorrect ignition timing | Adjust timing by rotating the magneto or adjusting the points baseplate. |
Timing the Ignition
The precise timing of the spark is paramount for optimal engine performance. The back plate assembly allows for adjustments to ignition timing. Typically, this is done by rotating the entire magneto (if it's a magneto ignition system) or by adjusting the position of the points baseplate relative to the crankshaft. The goal is to have the contact breaker points open and the spark occur just before the piston reaches Top Dead Centre (TDC) on the compression stroke. The exact timing will vary depending on the specific Villiers engine model, so consulting a workshop manual is highly recommended.
Conclusion
The Villiers back plate and its associated ignition components are critical to the reliable operation of these classic engines. By understanding how the ignition coil and contact breaker points function, and by performing regular checks and maintenance, you can ensure your Villiers-powered machine continues to provide many years of service. Remember to always use the correct parts and consult specific workshop manuals for the most accurate procedures. A well-maintained ignition system, centred around the back plate, is the key to unlocking the full potential of your classic Villiers engine.
Frequently Asked Questions
Q1: How do I know if my Villiers ignition coil is definitely faulty?
A1: Besides checking the resistance values as described, a visual inspection for any signs of melting or damage to the insulation can also indicate a fault. If the resistance readings are significantly outside the specified range, the coil is likely bad.
Q2: What is the correct gap for the Villiers contact breaker points?
A2: While 0.015 inches (0.38 mm) is a common setting, it is essential to refer to the specific manual for your Villiers engine model, as this can vary slightly.
Q3: Can I use a modern electronic ignition system with a Villiers back plate?
A3: In some cases, yes. Aftermarket electronic ignition conversion kits are available for many Villiers engines, which can replace the points and condenser, offering improved reliability and performance. However, this often involves modifying or replacing parts of the original back plate setup.
Q4: My engine runs, but the spark is weak. What should I check first?
A4: Start with the contact breaker points. Ensure they are clean, properly gapped, and that the rubbing block is making good contact with the cam. Then, check the ignition coil and its connections.
Q5: How often should I check the contact breaker points?
A5: It's good practice to check them annually or every few thousand miles, depending on usage. If you're experiencing ignition issues, check them as a priority.
If you want to read more articles similar to Villiers Back Plate: A Detailed Guide, you can visit the Automotive category.