remember DC electricity must travel in a circle (+)--->(-)
Therefore you should:
hook up a voltmeter to the large terminal on the starter solenoid with the battery switch on, is the large terminal that has the large positive cable attached to it measuring battery voltage?
hook up a volt meter to the yellow/red wire on the starter solenoid when someone turns it to start, what voltage are you getting there?
inspect battery terminals first if you use wing nuts on the battery terminals get rid of them, they will loosen and cause problems, use locking nuts instead or bolt on regular size marine grade batter clamps (what I did)
inspect the battery ground stud on the engine bell housing behind the exhaust hoses
if yellow/red wire voltage is lower than voltage at the large B+ terminal on the solenoid...
check the starter relay terminals
igniton switch, check for battery voltage at B terminal, (red wire) turn igntion on, check for voltage at the I terminal (purple wire), should be the same as batt voltage, then turn to start do the same check on the S terminal (yellow red wire)
there is also a large cannon plug connnector that connects the engine harness to the dash/gauge wiring, this has like 9 or 10 terminals, you may have to gently pull this apart and check for corrosion inside.
