Using an appropriate valve spring compressor, compress the valve springs. Lift out the keepers with needlenosed pliers, release the compressor, and remove the valve, spring, and spring retainer. Clean the valve stem with lacquer thinner or a similar solvent to remove any gum and varnish. Clean the valve guides using solvent and an expanding wire-type valve book cleaner. Replace any parts that are damaged or worn.
Place the spring on a flat surface nest to a square. Measure the height of the spring, and rotate it against the edge of the square to measure distortion. If spring height varies (by comparison) by more than 1.5mm ( 1 / 16 in.), or, if distortion exceeds 1.5mm ( 1 / 16 in.), replace the spring.
In addition to evaluating the spring as above, test the spring pressure at the installed and compressed (installed height minus valve lift) height using a valve spring tester. Springs used on small displacement engines (up to 3 liters) should be +/-1 lb. of all other springs in either position. A tolerance of +/-5 lbs. is permissible on larger engines.