Mike,
For clarity,
Automotive mechanical fuel pumps, if the internal diaphram leaks, there is a weep hole that allows the gas to drip down onto road service.
For all marine mechanical fuel pumps, there is a barbed fitting in that weep hole and the yellow hose attaches to that barbed fitting and also attaches to a barbed fitting in carb.
As Mark said, if Diaphram ruptures, gas will leak into yellow hose and flow to carb.
This will cause a excessive rich running condition. (Which you were reporting)
This is why its important to be able to see if gas is flowing. The black hose did not allow to see if gas was flowing.
If pump work correctly and no diaphram rupture, NO gas will be present in yellow line.