Re: Bench testing my combination fuel pump vacuum question

Posted by HH56 On 2009/8/30 17:11:01

Your assist theory is correct but pump is there for when the vacuum from intake manifold is low. The diaphragms move together but due to the springs, independently as the fuel side will sometimes not move at all or very little depending on how much and how fast the float is allowing fuel into carb. The springs pushing against the diaphragm also determine the output fuel pressure of 4-5psi.

The determining factor when the assist takes over or helps is the valving which works just as the fuel side does. When manifold vacuum is stronger, air is essentially just sucked through the pump and assist has little to do. When under open throttle or any low manifold vacuum conditions, the assist side is stronger and valving only allows exhausting into manifold side and suction from wiper side. While not as much volume or as strong as manifold vacuum, it is still sufficient to keep wiper motor moving --although slower.

Specs for those vacuum pumps in general were 8 1/2 inches hg minimum at 180 rpm and 12 inches hg maximum at 1800 rpm. As you see, lower than manifold but still sufficient.