I have a 1988 Formula with a 454 mag bravo 1 drive and a 177 Weiand running 4lbs boost at 6' with a mercruiser V6 module.
The problem I started experiencing is the engine cutting out at planning speed when I use the trim or tabs. The Voltage would drop down to 8 volts when using the trim on plane. The alternater checked out OK it charges around 13-14V below 2Krpm but drops to 12V above 3Krpm. My mechanic moved one of my starter cables from the engine block to one of the batteries. The engine no longer cuts out but the voltage still drops to 10 volts at planning speed when using the trim.
Does anyone have any ideas what may be causing this? the boat runs OK now but the "unknown" keeps nagging at me.

Thanks

Usually on my auto stuff, the alternator charges at 13-14 volts at anything over 2k. So the mechanic moved one of the ground wires back to the battery ground terminal? My first thought is bad ground somewhere or a bad cell in the battery. How old are the battery(s)?

Batteries are new that is the first thing I tried.
The boat ran fine the first 3 trips out, then I towed a guy in about 2 miles at idle. This fouled my plugs since I was at 7200'. I replaced them and started noticing the "cutting out". I also thought maybe a bad ground or bad alternator but the alternator checked out OK. I still think it should be putting out 14 volts at 4-5K engine rpm.

Have had similar problems. The battery and ground cables are just crimped. This allows corrosion. Cut a couple of inches of the cables (until you get to good non corroded wire) and replace cable ends. I soldier mine even though some say you should not and then use a good heat shrink (the kind with glue made in). Problem should go away.
Allen

Thanks
They look OK from a distance but I'll try it.