This type of engine still has tappet noise even after adjustment. This noise is said to come from the cams due to incorrect valve setup (when setting valve clearance each cylinder must be set to TDC) or from age and use of incorrect oil grade. The correct oil grade is 5W-30 semi synthetic oil. Another reason is the large tappet clearance on the exhaust valve. This could of course be reduced to lower the noise level but the engine would then suffer from a rough idle and usually stalled.