Back to your question, if:
There is fuel
There are not vacuum leaks in line from tank
Filter is clear
Pump is working
Then the pump would have filled the carburetor and you wouldn't see much fuel moving in a clear filter or a clear fuel line.
A good test would be to connect pump output to a line you can safely direct into a can where you can see what's coming out. Then power the pump with a 12VDC jumper. It should then pump steadily and move fuel in a steady unrestricted flow.
Please send us the Model Model and SPEC Code of the ONAN