Ford 351W Blowing main breaker

[ghall118]

I have a situation and could use some help. 1979 Carver 27' with a 351W engine and a Volvo outdrive. I replaced the distributor and cables for the distributor last year which was helpful. I replaced the 40AMP fuse on the the top/back of the engine block this year on general principals and because of my issue... My problem is this. The engine is hard to start, but once started it will run OK at idle (~1000rpm) at the dock for between 60 and 90 seconds then trip the 40AMP breaker that sits on the engine block which kills everything electrical on the boat. I can reset the breaker and restore battery power OK, but I have not tried to start the engine a second time. There are no other loads when I start the engine. What could be causing the breaker to trip? Any help would be greatly appreciated. Thank you

 

[makomark]

I'd say either the breaker has failed OR there is an excess current draw causing the breaker to open the circuit...maybe measure the current draw with the engine running....