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Posted by David Sherman on Sunday, August 24, 2008 at 22:22:21 :
In Reply to: Re: Fuel pressure adjustment shims. posted by The Dodge Boys on Sunday, August 24, 2008 at 17:57:31 :
I don't have a pump taken apart in front of me to look at, but I believe when the cam lobe pushes on the lever, the force acts directly on the pump diaphragm to suck fuel from the tank into the pump. Then when the cam lobe moves past the lever, the return spring inside the pump pushes the lever arm back to follow the cam, and pushes on the pump diaphragm in the right way to push the fuel into the carburetor. It has to be that way, because if the cam force directly pushed the fuel into the carb, it would build up extremely high pressure when the carb needle valve was closed. So, your "pressure regulation" adjustment in the fuel pump is the strength of that return spring. It could easily vary from pump to pump, either by design or due to weakening with age. Shimming the pump would mean that the spring didn't get compressed as far on the "sucking" stroke, and so it would push back with less force on the "blowing" stroke. But shimming would also reduce your pump volume, whereas having the right spring would maintain full volume while regulating the pressure correctly.
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