How to Adjust the Compression on a 21HP Kohler Engine

Turn off the 21 HP Kohler engine and remove the metal valve cover on the side of the motor. This cover lies to the side of the engine's spark plug. Remove the four bolts from the cover using a socket wrench, then pry the cover off with your hands.

Observe the two rocker arms beneath the Kohler's valve cover. There is one rocker arm at the top of the area beneath the cover, and one at the bottom. Each features a bolt, and at the center of each bolt is a screw. The engine's compression can be adjusted via these rocker arms. Loosen the screw in the bolt on the top rocker arm using a Torx screwdriver.

Use a feeler gauge as a guide for adjusting the top rocker arm. Attempt to insert the feeler gauge under one of the sides of the rocker arm. If you cannot insert the feeler gauge at all, use a socket wrench to loosen the rocker arm's bolt until the feeler gauge can barely slide under the arm. It should be snug. Conversely, tighten the rocker arm's bolt with your wrench if the feeler gauge too easily slides under the arm and can move about freely. Tighten the bolt until the feeler gauge slides snugly under the arm instead.

Retighten the Torx screw on the top rocker arm with your Torx screwdriver.

Alter the compression of the engine further by adjusting the lower rocker arm in the same manner as you did the top, using the feeler gauge as a guide. This will correctly adjust the engine's compression.

Set the valve cover back into place on the engine and reinsert its bolts. Tighten the bolts with your socket wrench.