I'm kind of baffled since it switched engines. Electrical short for sure, but who knows where.
He will need to address it with a volt meter when it is in the no crank/no start condition.

Check Your Main Harness connections ( big black round plug , , People have a tendency to over tighten the hose clamp. The pins in the male side of the connector will crush and flatten out , loosing connection with the female socket.
If the slit in the Male Pins are closed up . Take a small pocket screwdriver and open them up slightly.

Yes, I am leaning more towards electrical at this point as well. I am hoping to get out there today to dig into a bit further. To clarify, the key switch stops working until the solenoid is jumped once, then it returns to normal operation.

The engines are grounded together, I have dual batteries with a single Perko switch. when underway is placed in ALL. While moored or beached it is placed in #1 or #2 only. No issues with trimming tabs or drives. I'm super curious to figure this one out mainly because the issue changed sides within a week.

Griff,

Same here, at first I thought it was due to some type of safety switch issue (oil, temp, or emergency cut off) since the key switch became inop. with no gauge action either when turned.

1 switch/ 2 batteries per engine ? So total of 2 switches 4 batteries ?

or what it appears 1 switch /2 batteries total for the boat ?

If #2, how long has it been that way ? How long have you owned this boat ?

454 Mag with a Crane 132561 flat tappet cam, Crane Gold roller rockers, GM high-rise dual plane intake (some had single plane Weiand Team-G manifolds but the dual plane will be better all-around... can't beat the RPM Air Gap on the 454), Holley 800 carb, bigger stainless flame arrestor, finned valve covers, Gil exhaust.

The 420 is old technology. A hydraulic roller cam would be money well spent as would be better a better set of heads. Just my opinion.

Distantthunder's boat is a time capsule.

Brother, I think it's probably in the barrel connector. Have you tried opening up the pins some? Grounds would be good to look at, too.

SB,

1 Switch/ 2 Batteries Total for the whole boat. It's been like this since brand new, the boat is factory original with no modifications to the electrical from what I can see. I purchased it a couple weeks ago. As TomZ mentioned, Time capsule.

Waiting for this last band of showers to pass then I am going to open her up.

I am also thinking it could be the plug or pig tail where factory harness meets motor. I just had this exact thing happen when the new motors went in. Found out it was the pin slid out in the pigtail plug. ordered brand new ones from merc and it works like a charm. Im not saying thats what it is but when you get the time thats where i would look.