Greetings,I have a 33Ft 1990 scarab w/ twin 454 motors,my starboard motor looses power at WOT when I turn on my accessories i.e. lights,radio etc,Changed the battery and alternator and at 1500 rpm reads 14.2 volts at the battery terminals but my voltmeter on my dash reads only 8-10.Port engine and voltmeter works fine .Radio also seems to cut out if I crank it or when I start my motors?Not sure its a faulty ground or voltage regulator?

You have a corrosion issue. Suggest start with battery cables at both ends, and look under the insulation. Clean everything up shinny, then pull the 10 pin engine harness connectors and clean. Follow up behind helm to the buss bars

Many thanks to all,it was a loose main motor harness that was loose but still snugly wrapped up,re clamped it and power galore !. Unfortunately while driving happily my port engine cut out shortly thereafter and lost power to the coil.Now I have to figure my other motor electrical problem: