[G.M.]

The shim will take the free play out of the rocker arm which is lost motion and with free play you lose pump action. I can fine tune the pump output pressure with an adjustable push rod. I like 4 lbs cold down here where it's warm all year. As the pump gets hot the pressure drops. With this new gas if you keep the pressure up it reduces the chance of vapor lock. In 95 degrees plus the pressure will drop down below 2 lbs. When it get to 1- 1/2 the pressure starts to jump up and down and that is the edge of the engine starting to run ruff. When the engine is shut down the pump will be in the 135 degree range and this causes a full blown vapor lock as the engine and pump temperature rises after shut down and the engine won't start until the pump cools or is cooled with cold water or a plastic bag full of ice. I wouldn't just install shims as to thick will ruin the diaphragm. I know when I adjusted the pushrod I/2 turn it was making a noticeable change. Rebuild the pump to get rid of slop in the rocker arm. This way the pump starts to pump as the cam turns. Warn the rocker arm is moving but not moving the diaphragm so you may only get 1/2 the action of the pump. G.M.