The long arm rides on top of the spring, and the spring rides on top of the diapham. While trying to move the arm by hand to test the pump you will not be able to excert enough force to move the arm. Try wrapping the arm in a towel, then put the arm in a vise. You should then be able to test the arm movement by forcing the pump body.
The large spring is what compresses the diaphram to pump the fuel. The large arm only applies force to the spring. In situations where the carb dosen't need gas ( carb neddle valve closed), the large spring will just compress when the large arm pushes it down. The spring compresses because no gas is leaving the pump, so the diapham won't compress. Because of this something has to give, and that's the spring. By looking at how the pump works, it should help when re-assembling one. Hope this helps, but it easier when the part can be seen.
Posted on Jun 26, 1999, 9:00 AM from IP address 209.240.200.118