It's something in your batteries/wiring- to confirm start engines (one side and battery at a time) and then turn battery switch off. Carb 502 Mag should keep running with batts off, if you get the same reaction when hitting trim switch, horn, etc I think you rule everything else out. Idle should be fine.

Alternator should run that engine without battery but any switch being activated affects voltage going to ignition and hiccups. Not sure if running electric fuel pumps.

I shut my batteries off when i'm done for the day. Nothing works with them off. The dash is dead. How could the engine still run?

To everyone on this thread and to all in the past, I just want to say I really appreciate all the input and information from everyone here at Offshoreonly.com. I am glad to be a member. You have helped me many times in the past and now with my friends boat. Without this site (offshoreonly) and others it could a nightmare finding out these problems. Very much appreciated
Thank you

35fountain

sorry, meant start engine and then switch batteries off- engine should still be running ...

Pretty sure that is a big no,no.

purple wire will get no power from the ignition switch(red wire from starter/battery) and engine will shut off.

Agree, disconnect the battery from a running motor and your going to need another alternator

so did you find the problem?

Not yet but I'm pretty sure we're getting close just wanted to voice my appreciation