My boat never starts very well, especially when it is hot. It is a bbc, and I only run one battery. One thing I noticed is that all of the wire from the battery, and solonoid to starter was only #4 heavy strand wire, with some really light wieght ends/terminals. So, today I replaced all of the wire with 2-0 welding cable, and super heavy duty ends. It is still winter here in Wisconsin, but I did dry fire the boat to see if the new wire helped. All I can say is WOW! the motor crancks over like never before. All the new wire is so heavy that it wil proubly slow down the boat, but It sure will be nice haveing it turn over properly instead of barely even being able to spin the motor at all.
