Greetings
Hope someone can help me
I have 1990 scarab excel 33 with twin 454 mercruisers motor, when I turn my batteries on ,my port motor gets power right away ;but ,my starboard does not until I troll for 20 minutes on one motor then it powers ups ,starts up no problems ,runs without problems.Changed both batteries which are showing 12.8 V on both/interchanged both batteries both batteries and the same thing?the port batteries powers up my cabin and radio which I use very little.Also,I changed both alternators last year .Thanks

To see if it’s that motors battery switch, put both + cables on that switch to the same terminals as each other as to couple the cables bypassing the switch. If that works, get a new switch and put cables back to normal.

Have you ever disconnected the engine harness cannon plug ? Do slight ckeaning to terminals, use pocket screwdriver to very slightly spread male terminals and reinsert ? Is there a hose clamp holding cannon plugs on tight ? Btw: don’t crank down on this hose clamp, firm is good. Hand socket driver good here or 1/4 ratchet and just hold the head only while tightening. Hose clamp sockey size is 5/16” like all other hose clamps on engine.

voktmeter is your friend. Ck power at ign switch / glass fuse ends / etc/etc

Btw: can you describe ‘power up’ more exactly ? Do you leave that key ‘on’ , things don’t work, and then 30 minutes later they do ? What things ?

Thanks for responding,
What I mean by powering up is that the radio/gps/power to the key just turns on ;prior to that ,there absolutely no power the starboard side including the wire harness.The harness nestle on the motor with a plastic plate that keeps it from moving.I do think changing the battery switch may be a good idea .To add information ,I bought a smaller battery (27D)800CCA (had limited supply)vs my port battery 29D 840CCA which may not push enough voltage to get thru the resistance of the wires?

As stated , use a VOM or even a test light as a last resort to check voltage. I usually check working backwards from the starter stud then the cable end bolted to the starter . If you have a set of jumper cables go directly from a know good batter to the starter stud directly. If it fires up you've narrowed the issue down. My guess is a bad connection , bad cable or battery with a internal crack.

I would sometimes run into a similar issue with my 292 Fastech. I would have to flip the battery parallel switch on and then both would fire up. I’m not to familiar with your boat, but if you have a battery parallel switch I would use that.

Ground Wire Between Engines, Battery Switch Faulty, or Dead Cell in Battery. Keep it simple.

If there isn't one already, try connecting a separate ground cable between both Port/Starboard engines.

This happened to me a couple of times until we figured it out. After running out on Lake Michigan 10 miles down the shoreline, shutting it down for a short time, and then go to start it back up only to get a very sluggish and slight rollover with no start---like the symptoms of a dead battery----.I would keep trying every 5 minutes or so until it would suddenly start up again as if there was no problem about 20-30 minutes later----a very helpless feeling when needing to get back to port safely. It would usually happen after shutting them down after a run and then go to restart again---scary!

Found the problem ,main +battery terminal was corroded not visualy but change the cable and fitted a new battery terminal the problem went away
Thanks to all that replied