market dynamics
Analysis and exclusion of abnormal sound of valve mechanism of tappet machine of CG straddle motorcycle
Release date:2022.09.28

CG crossover motorcycle is a low-grade business vehicle designed earlier by Honda. It has the characteristics of stable performance, reliable work, compact structure, cheap price and convenient maintenance, etc. It is widely popular in the market and has been favored by the majority of motorcycle users for many years. The vehicle is equipped with a single cylinder four-stroke 157FMI natural air-cooled engine, which adopts the undermounted CAM valve mode and camshaft tappet transmission mechanism. Because of its structural reasons, camshaft tappet transmission mechanism after a period of time, will issue abnormal sound, to motorcycle users and maintenance personnel to troubleshoot a lot of inconvenience.

CG work process of valve-train is: on the engine crankshaft timing gear engages with the camshaft gear, through the transmission of the rocker arm and tappet and the radial component of the valve rocker arm as up, down, to realize the intake and exhaust door switch, in order to meet the requirements of the engine valve timing, make the engine run continuously.

In order to maximize the control of the axial movement of the crankshaft and the axial thrust brought by two pairs of helical gears (i.e. crankshaft timing gear and camshaft gear), an axial thrust device is installed on the periphery of the bearing hole of the left crankshaft box. The axial thruster is composed of a top pin, a spring and a pin body. When the crankshaft moves in an axial direction, the spring in the axial thruster can slow down the axial thrust of the crankshaft. In this way, the axial clearance of the crankshaft is controlled within a certain range, and the axial thrust and impact noise caused by the crankshaft timing helical gear and camshaft helical gear are also buffered. Therefore, when maintaining the engine crankshaft mechanism, we must pay attention to the axial clearance here, and whether the axial thruster that controls the axial clearance of the crankshaft is installed in place. If the spring of the axial thruster fails, the crankshaft will produce abnormal sound during operation, and the axial thruster assembly should be replaced in time. Under normal circumstances, the motorcycle should be checked once every 15,000 ~ 20,000 kilometers (open the left cover, remove the rotor of the magneto to check), and be replaced according to the situation.


Due to the particularity of the lower CAM valve mechanism, abnormal sound of the valve mechanism should be completely excluded. The following procedures can be referred to for inspection and treatment:

1. In the lower CAM valve mechanism, because the push rod reciprocating movement is not coplanar, it will give the rocker arm an additional axial moment, increasing the rocker arm's tendency to move back and forth. Therefore, in the rocker arm assembly on the cylinder head, the axial clearance between the valve rocker arm and the rocker arm support is strictly controlled at about 0.05mm, not more than 0.10mm. Disassembly inspection, can be used 0.05 ~ 0.10mm feeler piece measurement, if the gap is too large, will cause running noise, should be replaced according to the situation.

2. Check the clearance of inlet and exhaust valves. Unscrew the two screws on the left crankcase cover, use the sleeve plate hand to turn the crankshaft, so that the "T" mark on the flywheel is aligned with the timing inspection hole of the left box cover, and observe the two tappers can be rotated with fingers, indicating that the cylinder is in the position at the end of the compression stroke. In this case, insert the 0.02mm thickness of the feeler gauge piece between the valve gap adjustment screw and the valve rod end. If the gap is too large, it can be adjusted properly through the valve gap adjustment screw and the adjustment nut until the standard value is reached.

3. If the air distribution mechanism still has abnormal operation sound through the above adjustment, the rocker arm mechanism can be checked. There are two lower rocker arms, respectively controlling the intake and exhaust rocker arms, when checking, should carefully examine the rocker arm and push rod contact φ6mm, deep 2.5mm concave groove, if the shape is irregular, that there is partial wear phenomenon. The root cause may be that the shaft hole of the lower rocker arm is not parallel to the axis line of the camshaft, causing the 12mm hole of the lower rocker arm shaft on the cylinder body to be ground into an oval shape. From the outer circular surface of the pin shaft, if there are obvious indentation and bright spots, that is, the cylinder body rocker arm shaft hole processing error, should replace the cylinder body total. If the valve tappet has bending deformation, it will cause the push rod to rub against the machine and emit a different sound. When checking, the push rod can be placed flat on the glass plane, and the feeler stick in the gap during rolling, if more than 0.10mm, it will be replaced.

4. When replacing the cylinder body, attention should be paid to the right end of the two lower rocker arms equipped with waveform spring gasket. Its role is to offset the channeling noise of the lower rocker arm. If the axial hole plane of the lower rocker arm in the cylinder body (that is, the depth dimension of the elastic elastic washer) is processed too deep, the elastic action of the elastic elastic washer will disappear, and the axial impact will occur when the lower rocker arm swings up and down, causing abnormal sound. For this, two wave spring pads can be installed to compensate.

5. In the valve mechanism of CG model, the camshaft is arranged on the upper left side of the crankshaft box. Due to design reasons, the tooth surfaces of the camshaft gear and the timing gear on the crankshaft are not heat treated, which belongs to the soft tooth surface state. In the process of disassembly, attention should be paid to check the tooth surface of the two gears, without any knock and scratch, and the radial runout value of camshaft gear should not exceed 0.04mm, otherwise, it will cause meshing noise with the crankshaft timing gear. After REPLACING THE camSHAFT assembly, you can rotate the crankshaft while paying attention to whether the crankshaft is relaxed and tight in the process of rotation. Once the fault is confirmed, replace it with a new one and reinstall it.

6. Remove the camshaft check the CAM lift, if there is a clear line contact trace, is likely to be under the rocker shaft hole with R arc surface is not parallel, or under the rocker arm shaft hole and install the center of the CAM axis is not parallel, or CAM and CAM gear hole obvious deviation and not parallel, can be replaced by the rocker arm, and CAM gear components or cylinder to identify.

7. The small oil pool on the left side of the left crankcase is designed for lubrication of the camshaft. During inspection, it should be confirmed whether the lubricating oil in the small oil pool is clean, and the pin body on the left side of the left crankcase can be removed to drain the oil in the pool. When reloading, it is necessary to fill the small oil pool with clean oil to facilitate initial lubrication of camshaft and crankshaft timing gears. And pay attention to the daily maintenance, do not forget to replace the lubricating oil in the small oil pool.

8 valve spring function is to ensure that the valve and valve seat reliable seal, prevent the valve in the process of opening and closing due to the role of inertial force to produce abnormal "beating" or loose. Because the valve spring is mostly made of high carbon manganese steel or nickel-chromium manganese steel wire, after cold winding and forming, the two ends of the valve spring are ground flat, so the vertical degree of the valve spring is required to be higher, generally not more than 3°. If the verticality of the valve inner spring and the spring outside the valve is out of order, and the tilt of the two springs is just not in the same direction, the inner circle of the inner spring and the inner circle of the outer spring will interfere and cause noise. When checking the verticality of the valve spring, the inner and outer springs should be placed on the same plane (flat glass can be). Rotate the inner and outer spring at the same time, hold the feeler piece to measure the maximum gap value of the opposite surface of the two springs (generally not more than 1.5mm), divided by 2, that is, the inclination of a single spring, and finally calculate the perpendicularity of the spring according to the actual length of the valve spring. When detecting the elasticity of the valve spring, you can find a piece of the actual compression size of the valve spring and the thickness of the spring clamped on the pliers at the same time. After 24 hours, the deformation of the spring length shall not be greater than 0.15mm.

(Pictures and text are from the Internet, please contact to delete any infringement)