Guys !
i need your help!
im good at many things but f#%k all in electricity.. so i was running and stopped my 500 efi... one would not start back. Like nothing, completely dead. No click , nothing...

so i had a spare ignition, tested the new, did not work, so i put the original back
then changed kill switch, nothing again
so i put original back
came back home limping
next morning i try to start and it works !
so i changed the solenoid on the back top of engine. Today i went boating and started fine
then i shut the engines off, and same would not start again since it was warm again...i put my gator clip jump starter on the changed solenoid, and starts fine no problem

so where should i look next? Solenoid direct on starter?

I’m not being facetious, make sure it’s in neutral. I’ve done this more than once.

both neutral switch wires are twisted together
already... no chance for that
thanks! Need to find something else !

I'd try measuring the voltage at the starter solenoid where the wire from the key switch connects while turning key to "start". May be getting insufficient power from key switch.

Even if i already tried a new key switch?

Could it be as simple as the starter is getting old and is heat soaking (operates fine cold then inoperable when hot) - NOT as common in boats as cars from the muscle car era

Started to think that also
but if that is the case, why, even when still warm 1 min after not starting, it starts right up when jumping the solenoid on engine....???

What’s a gator clip jump starter ? And to witch solenoid wire or both ?

Yes. Key switch may be OK, but wiring/connections between the key switch and solenoid may be compromised leading to an excessive voltage drop. Issue could be before the key switch. Start taking voltage measurements at starter solenoid and if low, start working your way "upstream" until you find where the excessive voltage drop occurs. The neutral safety switch wires being "twisted together" may be causing a voltage drop too. All elec connections should be crimped or soldered.

Also good to check eng grounding.

Drawing out the start circuit may help troubleshoot. Do you have 2 solenoids for each eng? One on top of eng and one on starter??

Your picture is the slave solenoid. There is another one on top of starter.