It's a question of making the fuel pump suck harder to get fuel. The normal way to route the fuel is primary - pump - secondary, and the reason is that the fuel pump pulls fuel out of the first filter, which has coarser filter element and is less restrictive, then pushes pressurized fuel through the second filter, which has quite fine element and is a lot more restrictive. The first filter is kind of a trash, algae and water trap filter. The second filter is the real fuel filter. The sensor mounted on the secondary filter head is a thing (pressure sensor switch? I don't know what it's called either at this senior moment in time) that doesn't let the starter motor crank if there is fuel pressure at the secondary filter.
Edit: My manual calls the first primary filter a "fuel strainer", not a fuel filter. The switch is called a fuel pressure switch, and it provides ground to the starter solenoid, opens between 8 and 10 psi.
On my MCI there are additional fuel lines on the heads - pressurized fuel goes into both heads as you show, and the return from the left (driver's side) head goes into an "in" port on the right hand head. Return from the right hand head comes off the back of the head, goes through a restrictor and back to the fuel tank. So the right hand head has two hoses connected to "in" ports which confused the heck out of me for about a day when I was first doing the fuel line plumbing when I did my re-power.
My priming pump is just mounted in-line with the main fuel line prior to the first fuel filter. The main pump pulls fuel through it just fine. I turn it on, let it run till it's sound changes so I know the lines and filters are full (maybe 10 seconds) then I turn it off. If I leave it on the engine runs fine, the the pressure trips that pressure switch so my engine won't crank. Caused major panic attack the first time I used it when I left the pump on and could only fire the engine once!
Thanks for the info. I suppose it's working pretty well so maybe i'll keep this setup untill I have a reason to switch over. Though the stress on the pump could be enough reason.
Built a little T-valve with a butterfly and some fittings, did the garden sprayer thing, 1.5 gals, squirt with some starting fluid and it cranked right up. Though it won't air up over 80 (takes 2-3 mins) then just sits. Hmm. Time to figure out the next issue!