market dynamics
Analysis and elimination of abnormal sound in valve mechanism of tappet machine of motorcycle
Release date:2024.03.14

CG straddle motorcycle due to its structural reasons, camshaft tappet transmission mechanism after use for a period of time, will make abnormal sound, to motorcycle users and maintenance personnel troubleshooting bring a lot of inconvenience.

Due to the particularity of the lower CAM valve mechanism, to completely eliminate the abnormal sound of its valve mechanism, you can refer to the following procedures for inspection and treatment:

1. In the lower CAM valve mechanism, due to the influence of non-coplanar reciprocating movement of the push rod, it will give the upper rocker arm an additional axial torque, increasing the tendency of the upper rocker arm 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 and not more than 0.10mm. When disassembling the machine for inspection, the feeler plate can be measured with 0.05 ~ 0.10mm. If the gap here is too large, it will cause running noise, and it should be replaced according to the situation.

2. Check the clearance between inlet and exhaust valves. Unscrew the two screws on the left crankcase cover, turn the crankshaft with the sleeve plate hand, so that the "T" mark on the flywheel is aligned with the engraved line of the timing inspection hole of the left case cover, and observe that the two tappets can be turned with your fingers, indicating that the cylinder is in the position at the end of the compression stroke. At this time, the feeler plate with a thickness of 0.02mm is inserted between the valve clearance adjustment screw and the valve rod end. If the gap is too large, it can be properly adjusted through the valve clearance adjustment screw and adjustment nut until the standard value is reached.

3. If through the above adjustment, the valve mechanism still has a running noise, you can check the rocker arm mechanism. There are two lower rocker arms, respectively control the intake and exhaust rocker arms. During inspection, carefully check the Φ6mm and 2.5mm deep concave groove that the lower rocker arm contacts with the push rod. If the shape is irregular, it indicates that there is partial wear. The root cause may be that the shaft hole of the lower rocker arm is not parallel to the shaft 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 point of view of the outer round surface of the pin shaft, if there are obvious indentation and bright spots, it means that the cylinder body rocker arm shaft hole is wrong, and the cylinder body should be replaced. If the valve tappet has bending deformation, it will cause the push rod to rub against the machine and make a noise. During the inspection, the push rod can be placed flat on the glass plane, and the piston plate is plugged in the gap during the rolling process. If it exceeds 0.10mm, it will be replaced.

4. When replacing the cylinder body, it should be noted that the right end of the two lower rocker arms is equipped with a waveform spring pad, its role is to offset the sound of the lower rocker arm when it is working, therefore, when the lower rocker arm is reassembled, the first can not be omitted, and the second direction can not be reversed. If the shaft hole plane of the lower rocker arm in the cylinder body (that is, the depth size of the corrugated spring pad) is processed too deeply, the elastic effect of the corrugated spring pad will disappear, and the axial impact will cause abnormal sound when the lower rocker arm is swinging up and down. For this, two wave 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 two tooth surfaces of the camshaft gear and the timing gear on the crankshaft are not heat treated and belong to the state of soft tooth surface. In the disassembly process, attention should be paid to check the tooth surface of the two gears, there should be no bump and scratch, and the radial runout value of the camshaft gear should not exceed 0.04mm, otherwise, it will cause the meshing noise of the gear timing with the crankshaft. After replacing the camshaft assembly, you can rotate the crankshaft while paying attention to whether half of the crankshaft is relaxed and half of the crankshaft is tight during rotation. Once the fault is confirmed, it should be replaced with a new part and re-assembled for trial.

6. Remove the camshaft to check the lift part of its CAM. If there is a more obvious line contact mark, it may be that the lower rocker arm shaft hole is not parallel to its R arc surface, or the lower rocker arm shaft hole is not parallel to the central axis of the installation CAM, or the CAM body and the CAM gear hole are not parallel, which can be identified by replacing the lower rocker arm, CAM gear assembly or cylinder body.

7. The small oil pool on the left side of the left crankcase is designed for lubricating the camshaft. During the inspection, it should be confirmed that the lubricating oil in the small oil pool is clean, and the pressure pin body at the left lower end 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 the initial lubrication of the camshaft and crankshaft timing gear. And pay attention to the daily maintenance, do not forget to replace the lubricating oil in the small oil pool.

8. The function of the valve spring is to ensure the reliable sealing of the valve and the valve seat, and to prevent the valve from abnormal "beating" or loosening due to the action of inertial force during the opening and closing process. Because the valve spring is mostly high-carbon manganese steel or nickel-chromium manganese steel wire, the two ends of the cold winding and then ground flat, so the verticality of the valve spring is higher, generally not greater than 3°. If the verticality of the inner spring and the outside spring of the valve is out of whack, and the inclination of the two springs is just not in the same direction, the outer circle of the inner spring and the inner circle of the outer spring will interfere and cause the rubbing 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 springs at the same time, hold the ruler to measure the maximum gap value of the opposite surface of the two springs (generally not more than 1.5mm), and then divide by 2, that is, the inclination of a single spring, and finally calculate the verticality of the spring according to the actual length of the valve spring. When detecting the spring elasticity of the valve spring, an iron block with the actual compression size and thickness of the valve spring of the model can be clamped on the vice at the same time, and the deformation of the spring length shall not be greater than 0.15mm.

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