In old and out-of-print motorcycles, the contact points determine the ignition timing of the spark plugs that explosively burn the compressed air mixture in the cylinder. The spark advancer is a device that works to achieve the ideal combustion at a wide range of operating speeds from idling around 1,000 rpm to over 8,000 rpm.
A certain amount of time is required for the mixture in the combustion chamber to explode and burn
Whether it is an out-of-print motorcycle or the latest model, the ignition timing is very important in an internal combustion engine where the spark plug ignites the air mixture in the cylinder. When the intake and exhaust valves are closed and ignition sparks fly into the combustion chamber where the mixture is trapped, explosive combustion occurs, pushing the pistons down and turning the crankshaft vigorously to generate engine power.
When the piston moves back and forth in the cylinder, at what position does it send out sparks? That is the ignition timing. When you burn wood in camping or light an oil stove, it will not suddenly reach maximum heat. It takes a certain amount of time for a small seed flame to spread. The same phenomenon occurs with the air mixture in the engine. In other words, it takes a certain amount of time for the air mixture to spread after a spark is ignited by the spark plug.
Camping firewood can wait for us until the heat is large enough, but this is not the case with an engine. The time it takes for a small seed fire to spread to a large flame is constant, but the engine's piston speed increases as it revs up.
You're probably wondering what I'm talking about, right?
There is a close relationship between the timing when the air mixture burns and expands and the timing when the piston moves from top dead center to bottom dead center. For example, if explosive expansion occurs when the piston is on its way to the top dead center of compression, it will be a force that pushes the rising piston down. Conversely, if explosive expansion occurs while the piston is descending past top dead center, it will not force the piston down.
Since combustion requires a certain amount of time, the ignition timing is set so that the mixture is ignited before the piston reaches the top dead center of compression and reaches maximum pressure after passing the top dead center.
Ignition timing can be set according to a specific rotation speed, for example, 1,500 rpm, if the engine is used at a basically constant rotation, such as the engine of a generator. In the case of a generator, a device called a governor is installed to prevent the speed from rising or falling depending on the increase or decrease in load.
However, in the case of a motorcycle, the engine speed varies from 1,000 rpm idle to 12,000 rpm and even higher depending on the model. The speed at which the piston moves back and forth also changes.
Some of you may remember learning the concept of angular velocity in high school physics. As the engine speed increases, the angle at which the crankshaft rotates per second increases. It is easy to imagine that the reciprocating speed of the piston is faster near the red zone than at idling. On the other hand, since the combustion speed is unchanging, the ignition timing that is optimally adjusted at idling will be delayed relatively as the engine speed increases.
It is said that the best performance cannot be achieved in an internal combustion engine with large fluctuations in the number of revolutions if the ignition timing is always the same.
- Point 1: When a gas mixture burns explosively, it takes a certain amount of time to reach maximum pressure.
- Point 2: If the ignition timing doesn't change, the ignition condition in the whole area from low revolution to high revolution may not be the best.
Optimum ignition timing varies with engine speed
This is where the ignition advance device is employed. In the old engines, the rider used to adjust the ignition timing manually. Since the non-contact system using transistors was adopted, the igniter automatically adjusts the ignition timing inside.
A spark advancer with contact point ignition is a mechanical advance adjuster that falls between manual and automatic. The manual advance is really a mechanism of the past, and the automatic advance has become popular in the point ignition motorcycle after 1950's. Incidentally, since the non-contact method of the transistor becomes popular at a dash after the latter half of the 1970s, the actual period of the point + automatic advance is about 40 years or less. Moreover, even in the vehicle equipped with the ignition system of the contact point type, there is a model without the advance device mainly in the small displacement vehicle.
The spark advancer consists of a weight that receives centrifugal force from engine rotation and a point cam that opens and closes the contact points. When the engine speed rises and the centrifugal force increases, the weights open outward, and the cam connected to the weights turns in the direction of the crankshaft rotation. The ignition timing is accelerated by this mechanical mechanism. This weight is pulled by a spring, and if the engine speed becomes low and the centrifugal force applied to the weight becomes small, it returns to the original position and the ignition timing also becomes late.
The relationship between RPM and ignition timing advance angle is set by the manufacturer according to the characteristics and specifications of the engine. For example, in the case of the Kawasaki Z1, the manufacturer's set values are 20 degrees before top dead center at 1,500 rpm and 40 degrees before top dead center at 2,350 rpm. On the other hand, the 750cc Z2 engine is similar, but at 1,500rpm it is set at 5 degrees before top dead center, and at 3,000rpm it is set at 40 degrees before top dead center.
The optimum value will vary depending on the bore stroke setting and camshaft characteristics, so it is best to follow the standard specification of each engine for maintenance rather than tuning.
Based on that premise, ignition timing at engine speed depends on weight of weight and tension of spring. If the spark advancer is stock, the advance characteristic will be as the motorcycle manufacturer's setting, but if the spring is broken due to aging, the weight will open by centrifugal force and the ignition timing will be early even if the engine speed is low.
On the other hand, if the cam doesn't move even if the rpm rises because of the poor lubrication of the pivot shaft of the weight or the point cam, the ignition timing doesn't advance and the power feeling at high rpm decreases.
- Point 1: The spark advancer decides the ignition timing according to the rotation of the engine by the balance of the weight and the spring.
- Point 2: The ignition timing at idling and after the advance angle is different according to the engine.
The basic maintenance of the Spark Advancer is to grease it up.
The points and spark advancer of the Kawasaki Z1 and Z2 are located on the right side of the engine. When the motorcycle was new, points ignition was the norm, but regular maintenance was essential. Today, there are non-contact ignition units that can be bolted on (the AS Utani SPII Full Power Kit is a well-known example), and many users are replacing their ignition units with these parts, which provide both a strong spark and maintenance-free operation.
(2) The contact breaker can be removed by removing the long bolt at the center that is fixed to the crankshaft. There is a locating pin on the contact surface between the back side of the contact breaker and the crankshaft to avoid confusion about the installation direction when restoring.
(3) The notch at the base of the point and the convex part of the weight engage and the movement is interlocked. If the notches are aligned and pulled toward you, the cam will be removed.
(4) Apply grease to the shaft where the cam contacts. There is a grease reservoir groove, so apply grease as much as possible without it sticking out from here.
Contact breaker point maintenance includes checking the point gap, roughness of the point surface, and ignition timing, but if the model has a spark advancer, it is also important to check its function.
As mentioned earlier, check that the weight and cam are not sticking and that there is no damage to the spring, but since the spark advancer is often located behind the points base, it is necessary to adjust the ignition timing again if the points base and spark advancer are removed.
The structure of the Spark Advancer is simple, and the relationship between the weight and the cam can be seen at a glance. During maintenance, pay attention to whether the weight opens and closes smoothly, whether the spring is broken or stretched, and whether the cam rotates smoothly. However, be careful not to apply too much grease, because if too much is applied, the scattered grease may adhere to the point surface and cause ignition failure.
You can check the timing light to see if the spark advancer is functioning properly. In the case of the Kawasaki Z, it is normal if the tick line on the crankcase matches the F mark on the spark advancer at 1,500 rpm and the pin on the spark advancer matches the tick line on the crankcase at over 3,000 rpm.
Even with the engine stopped, it is possible to adjust the idle speed equivalent, i.e. adjust the point base so that the points open at the moment the F mark and the tick line coincide, but it is not possible to visually judge whether the pin and the tick line of the advancer coincide at 3,000 rpm. Therefore, it is impossible to judge the function of the advance angle side of the spark advancer correctly without a timing light.
Because the number of the point ignition vehicle is an overwhelming minority among the existing motorcycles, I think that the majority of people do not understand it even if it is called a spark advancer. However, even in the case of transistor ignition models, where the ignition timing is determined electrically, the ignition timing is changed according to the engine speed. In addition, newer motorcycles combine information from multiple sensors to determine ignition timing based not only on engine speed but also on engine load.
In other words, regardless of whether the engine is new or old, there is no change in the natural order of things in that it takes a certain amount of time for the mixture to burn, and ignition timing control is important to achieve maximum engine efficiency within that time. Although the spark advancer is a simple combination of a weight and a spring, it contains an essential element for an internal combustion engine that automatically changes the ignition timing according to the engine speed.
- Point 1: Proper greasing is essential for the smooth operation of the spark advancer.
- Point 2: Modern engines also control ignition timing according to the speed and load conditions.
See Accessories of AS Uotani
See KAWASAKI Moto Index Page
See Accessories for KAWASAKI Z1 (900 Super Four)
See Accessories for KAWASAKI Z2 (750RS, Z750 Four)