The valve is only found on a four-step motor, while the two-step motor generally does not use a valve. The valve on the four-step motor is mounted on the cylinder head. The duty of the valve is to open and close the combustion chamber. Each cylinder is equipped with two types of valves (suction and exhaust). The opening and closing of the two valves are arranged with a shaft called the camshaft.
So the four-stroke motor cylinder requires two cams, namely the inlet cam and the exhaust valve cam. The camshaft is rotated by the crankshaft through a gear or chain transmission. The camshaft rotates at half the speed of the crankshaft. Thus, the diameter of the gear on the camshaft is twice the diameter of the gear on the crankshaft. Therefore the crank pen trajectory is half the camshaft trajectory.
The valve is made of a hard material and easily conducts heat. The valve receives high heat and pressure and always moves up and down, so it requires high strength. In addition, the valve should be resistant to heat and friction.
The function of the valve is actually to disconnect and connect the cylinder chamber above the piston with the outside air when needed. Because the process of combustion of gas in the engine cylinder must take place in a tightly closed combustion chamber. If there is even a slight gas leak, the combustion process will
be disturbed. Therefore the valves must be tightly closed when the gas combustion takes place.
The inlet valve and the exhaust valve are mushroom and are called the “poppet valve”. The inlet valve receives combustion heat, thus the valve experiences uneven expansion which will reduce the effectiveness of the valve seat density.
To increase efficiency, entry holes are usually made as large as possible. Meanwhile, the exhaust valve also receives heat pressure, the heat pressure received is higher, this will reduce the effectiveness of the density as well so that the effect on the valve seat wears easily. To avoid this, the clearances (clearance) between the valve stem and stem head are made larger.
To distinguish the inlet valve with the exhaust valve can be seen in both diameters, the diameter of the inlet valve is generally greater than the exhaust valve.
The valve head has a very important role because it must continue to work well, even if the temperature changes. The upper plane of the valve head is called a shield. The shape is concave and some are convex. The concave shield is called the trumpet shield and is usually used as an inlet valve. While the convex shield is used as an exhaust valve because of its higher strength.
The springs are also mounted on the valve. Valve springs are assigned to close the valve in accordance with the movement of the lever away from the end of the valve stem. Valve Placement Innovations
Various types of valves can also be distinguished from the way they are placed on the cylinder head. Motorcycle engine innovations are carried out to anticipate high speeds, increase output power, and make the construction effort as light as possible. There are three kinds of valve innovations in terms of placement, namely the Side-Valve, Overhead-Valve (OHV), and Single Overhead Camshaft (SOHC).
The side valve (SV) is the simplest and lightest construction and its mechanical drive is placed next to the valve. This model is considered the oldest and less able to serve high rounds. Therefore, this model was modified to become an OHV model. This type of valve has a longer valve stem because it is driven by a camshaft which is located parallel to the crankshaft. The camshaft movement is guided by pipes that are firmly attached to the cylinder block.
The third type (SOHC) is designed to make valve system components lighter. The valve stem is driven not by the camshaft, which is considered to make the component heavier, but through the gears. In fact, some of the latest innovations are driven by chains (cam chains). This latest innovation is called Double Overhead Camshaft (DOHC).
The compression pressure in the combustion chamber is greatly influenced by adjusting the valve gap. If the valve gap is smaller than the standard means the valve opens quickly and closes longer, a longer opening makes more gas enter. As a result, gasoline is more wasteful and the result of a delay in closing the valve is that the compression pressure is leaking because, during the compression step (when the piston moves from the bottom up), the valve has not closed when it should at the time the valve has to close tightly this results in reduced engine power. The engine cannot be stationary, and is difficult to start, other than that due to the valve gap is too narrow an explosion can occur on the carburetor.
Furthermore, if the valve gap is greater than the standard means the valve is late opening and closing fast. If this happens at the inlet valve, the intake of the air-fuel mixture takes place quickly so that the amount of the incoming mixture is small. Compression pressure is low because of the small amount of mixture of gasoline and compressed air. If the compression pressure is low it will result in reduced motor power. The next result is that the engine is difficult to start. Afterlife the engine noise is very noisy. Because the gas intake is lacking, the engine will falter at high speed. Meanwhile, the engine cannot spin stationary. That is why the valve gap must be adjusted correctly.
Usually, the gap between the inlet and exhaust valves is about 0.04 – 0.07 mm.
Check, adjust, and maintain valves:
a. Tuning one cylinder motorcycle valve gap
The ignition is OFF. The position of the piston on the compression top. To ensure that the piston position on the compression top, note that at this time the T mark on the magnetic rotor is exactly the same as the line mark on the motorcycle body, the platinum gap opens and both valves close.
If the piston position is not yet right in the compression top position, rotate the crankshaft with a lock. In order to rotate it lightly then remove the spark plug from the holder.
Adjust the valve gap with the feeler in accordance with the provisions. To adjust the valve gap, loosen the nut and insert a feeler with a thickness that is within specifications. After that, turn the adjusting bolt and harden the lock nut so that the feeler can only be pulled with a bit of resistance (a bit heavy). After the lock nut is tightened, insert the folder once again to check that the adjustment is correct.
After both valves are adjusted, reattach the removed part and turn on the motor for control. If it turns out that the valve gap is too loose there will be noise from the cylinder head. If the valve gap is too narrow it is usually difficult to start the motor.
b. Tuning two-cylinder motorcycle valve gap
The ignition is OFF. The position of the first cylinder-piston on the compression top. To ensure that the piston position of the first cylinder on the compression top, note that at this time the T mark on the magnetic rotor is precisely in line with the line mark on the motor body, platinum crack opening and the two first cylinder valves close.
If the piston is not in the compression top, rotate the crankshaft with the key. In order to turn it lightly, first, remove the spark plug from the holder.
Adjust the two-cylinder valves first as how to adjust the valves on a single-cylinder motorcycle. The other cylinder valve can be adjusted after the crankshaft has been turned one full turn from its position.
Issues to PLEASE NOTE:
If the adjusting screw is turned right clockwise then the valve gap becomes narrow. If the adjusting screw is turned to the left, counterclockwise, the valve gap is loose.
When hardening the adjusting bolt lock nut must be held to prevent the valve gap from changing.
Feelers that have worn out or are bent should not be used to adjust the valve openings.
Don’t harden the lock nut too hard because it will make it difficult to loosen it again. 5. To facilitate the adjustment of the valve, remove the offending parts, such as the gas tank for certain types of motorcycles.
Camshaft (Nokn As)
The camshaft is a device used in an engine to run a poppet valve. It consists of a cylindrical rod. Cam opens the valve by pressing it, or with other relief mechanisms when they spin.
The relationship between camshaft rotation and crankshaft rotation is very important. Because valves control the flow of fuel input and discharge, they must be opened and closed at the right time during the piston stroke. For this reason, the camshaft is connected to the crankshaft directly (through the gear mechanism) or indirectly through a chain called the “time chain”.
In a two-stroke engine that uses a camshaft, each valve opens once for each rotation of the crankshaft in this engine, the camshaft rotates at the same speed as the crankshaft.
In a four-stroke engine, the valves open halfway less, therefore two full crankshaft turns to occur at each camshaft rotation.
Slide friction between the cam face with the follower depends on the amount of friction. To reduce this wear, cam and follower have hard surfaces, and modern lubricating oils contain ingredients that specifically reduce sliding friction. The lobe (earlobe) of the camshaft is usually tapered, causing the follower or valve lifter to rotate slightly at each pressure, and wear out the components. Usually, the face of the cam and follower is designed to wear together, so when one has worn it must be replaced to prevent excessive wear.
Cam Chain and Stretch
The inlet and exhaust valves on the motorcycle open and close according to the process that occurs in the combustion chamber. The process that occurs in the combustion chamber
The motor is determined by the piston step where the piston step is determined by the rotation of the crankshaft. Conversely, the crankshaft rotation is also influenced by the process that occurs in the combustion chamber. Thus there is a reciprocal relationship between crankshaft rotation and the processes that occur in the combustion chamber
In order to open the valves in accordance with the process that occurs in the combustion chamber, the mechanism of opening and closing the valves is driven by the crankshaft rotation. There are three types of the valve drive mechanism, namely with a driving rod, gears, and chains (camshaft chain).
The motorcycle camshaft chain must be installed with sufficient voltage. Camshaft chains that are too tense will cause a whirring noise, especially at high turns while camshaft chains that are too loose will cause noise. To adjust it must first be considered the adjustment mechanism. The way to adjust the camshaft chain for each motorcycle is not the same.
If the chain tension changes, it will affect the engine speed, valve timing, or when the ignition will change too.
To produce standard chain settings, there are 3 types of chain settings:
1. Manual tuning type This type requires periodic tightness adjustment. How to adjust by pressing the pressure rod
2. Type of automatic adjustment If the chain is loose, then the pressure rod will automatically press the chain guide (rubber), because of the press. Rubber will bend and will press against the chain so that the chain experiences tension. Furthermore, the rachet-shaped pressure rod moves in the same direction and cannot return.
3. Semi-automatic type Chain tension automatically adjusts itself, if the locking bolt is released, so the pressure rod will go inside due to pressure.