I have a Motor Guide Brute 767, 67 lb thrust 24 volt trolling motor. It will only run at about half speed even with new batteries.

running to slow indicates voltage too low
you will have 3 X 12 volt batteries connected in series ( positive to negative to positive to negative with positive to motor The first battery negative connected to the ground on the motor
the multicore wire cable is for 12 volt draws as in fish finder, light etc and can be connected across any single battery terminals
They could also be for a foot controlled pedal that is forward and reverse operation
that will be explained in the user manual

Yes but not for long. You need to buy a deep cycle battery. Regards Phil.

This is most often caused by a faulty 24 Volt, section, (usually damaged output semiconductors, or the drive?) in the speed controller, fault, the only "Fix" is replacement. Simply obtain the entire speed controller and replace it "En Bloc" as it were. You will need to also obtain a service manual so you can disassemble it and obtain the parts numbers for the replacement parts.Otherwise take it to a professional and ask for a "Quote" as to prospective costings and go from there.

Hey Chris,, you are starting a project (or wanting to) that is going to be problematic... It will be easier to go buy a new trolling motor with the money you are going to spend attempting this conversion... the newer trolling motors are so much more efficient...
Trolling Motors Fishing Motors Electric Boat Motors MotorGuide Trolling...
https://www.minnkotamotors.com/

i think its time to strip this motor out,, it sounds like its has a short in the field coil winding,,as its pulling far to many amps,,,,

Just got parts 4 my 96 brute from trolling motor parts .com

trolling motor parts.com

the black wire goes to - and the red goes to + for 12 volts. If you want 24 volts you need to add another battery and run a jumper wire from the - of the first battery to the + of the second battery and hook the white wire to the - of the second battery. Here is a picture to help you.

12 Volt

You should only run the motor with a 24v battery or two 12v batters 'jumped' out. To jump the two batteries, connect the positive on one batter to the negative on the other battery and hook the trolling motor battery cables up to positive and negative terminals that are NOT jumped out as you would normally. Don't run this with 12v as you will have serious problems if you do. They do not make a 65lb thrust in 12v - too much pound thrust to run off of 12v.