1... Remember that a carb does not need pressurized fuel to operate. It draws in fuel to the engine by vacuum/venturi/Bernoulli effect, and by getting a shot from the accelerator pump. Therefore, even at zero fuel pressure the engine will start just fine if there is gas in the fuel bowl.
2... I wouldn't go to the trouble of wiring up a second hot feed to the fuel pump unless you experience extremely difficult starting. I would just take a wire from the starter solenoid over to the fuel pump so that when the key was in the START position, the fuel pump would come on but as soon as the engine starts the power would come via the oil pressure safety switch.
1... His Volvo Penta AQ260 is likely fitted with a Rochester Quadra-jet. If so, these are prone to evaporating the fuel bowl when sitting for any length of time. The momentary helm switch is the Cat's Meow for priming the fuel bowl prior to any engine cranking.
Key word: "Momentary" helm switch.
Why crank and crank when the momentary by-pass helm switch solves this problem?
You'll end up extending the life of the starter motor, and your battery bank will undergo a lessor amount of deep cycles.
Doesn't sound like much..... but multiply this over the years of long cranking required to get fuel into the fuel bowl.
2... A relay needs to be brought into the system in order to prevent a back-feed to the starter motor's solenoid S terminal.
Otherwise, once the pump is powered (via the oil pressure switch), power may continue to energize the S terminal via electrical current back-feed.
One slight snag I can see running a "prime" wire to the starter solenoid is... If the engine builds oil pressure before the engine starts then the oil pressure switch can back feed to the starter keeping the starter engaged even if you let go of the swith, it would need a diode to prevent this. Had the same problem trying to excite a 3 wire gm alternator without an oil pressure switch
Or.......... a relay brought into the loop.
The momentary helm switch powers the slave side of the relay..... not the solenoid circuit.
Conversely, the solenoid circuit also momentarily powers the slave side of the relay.
Once this circuit lets go of the relay, all returns to normal, and the fuel pump will continue to operate as long as there is oil pressure above the switch's NO (normally open) pressure setting.
Let me amplify/clarify a little.
This is on a 77 SeaRay Sundancer 25 with Volvo 280 outdrive ... I think I recall it refernced as AQ265? 265 hp chev 350 conversion.
I have 2 concerns/options regarding the physical placement of the new electric fuel pump.
I can place it low in the bilge to insure it is below the normal fuel level; the concern is that is before the fuel filter and subject to any existing crud in the fuel tank.
I can place it after the fuel filter (yes, all uscg certified fuel line and filter is not mounted on the engine); the concern is that the new pump would be a little higher and if there is too much restriction in the fuel filter that would affect fuel flow. Maybe I am just fretting here over nothing and this is the place to put the new pump. Because I have been around the ocean all my life I am very picky about details of my machinery before going out.
I have the low oil pressure switch, but had not thought about the start bypass switch until reading here ... good idea.
At least initially I intend to place a fuel pressure regulator and gauge somewhere before the carb.
Rod, I'd take that Volvo Penta OEM engine mounted fuel filter, and toss it in the recycle bin.
This OEM filter arrangement is not a good design by today's standards.
Instead, purchase yourself a Parker/RACOR system, and remotely locate it OFF Engine, and first in line immediately after the fuel tank pick up and anti-siphon vavle.
Use the latest USCG approved gasoline fuel hose.
BTW, I believe that you can find a mounting plate that allows the new electric pump to mount in the same location as the mechanical pump.
The plate blocks off the oil cavity, and of course the push rod would be removed.
As for a pressure regulator..... what makes you think that this is necessary? Just curious.
As for a pressure gauge installed in a gasoline fuel pump system, you'd best check to see this would be USCG approved.