* Incorrect valve lash. If the valve lash is too tight, it can cause the valves to open and close too quickly, which can lead to knocking. If the valve lash is too loose, it can cause the valves to float, which can also lead to knocking.
* Worn valve guides. Worn valve guides can allow the valves to move around too much, which can lead to knocking.
* Damaged valve seats. Damaged valve seats can prevent the valves from sealing properly, which can lead to knocking.
* Carbon buildup. Carbon buildup on the valves or piston heads can also lead to knocking.
If you hear a knocking noise after a valve adjustment, it's important to have the engine checked by a qualified mechanic to determine the cause of the noise and to have it repaired as soon as possible.
