Using a feeler gauge, measure the clearance in the valve drive mechanism, which should be (on a cold engine):
- intake valves: 0.26 - 0.34 mm;
- exhaust valves: 0.29 - 0.37 mm.
Compare the obtained values with the required ones.
Replace tappets, the gaps between which and the camshaft lobes are out of tolerance.
Measure tappet thickness with a micrometer and repeat for tappets with out-of-tolerance valve clearances to match tappet thickness.