Well...
This thread was about priming a mechanical fuel pump so I apologize for opening a scary box.
I wasn't trying to start a discussion about fluid flow, just curious why Kenth stated "there is NO suction going on in a mech fuel pump."
I'm confident he understands what makes fluids flow, same as you all have shared, and I'm also confident he knows how mechanical pumps work. I figured he was making a point, but wasn't sure what.... and given the topic about priming, I know he has experience to add.
I did try to suck fuel at the carb by pumping the fuel line with the MityVac THROUGH the pump. It did NOT work. Most likely, the vapor return line inside the pump doesn't allow a vacuum to build up unless there's a lot of airflow. That return line has a restriction but I think acts as an open vent back to the tank. I speculate a higher-flow vacuum pump would be able to overpower that vent restriction and pull fuel through the pump. That would truly prime the whole system up to the carb but might get pretty exciting when the fuel showed up. And in the end, the pump primes itself fast.
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