There's something called a "series resistance test" which involves measuring the voltage drop at every connection from the positive battery post to the starter, to the negative post. By using a low-reading voltmeter and connecting to each side of every connection, it's easy to spot the dirty, high-resistance connections. No connection anywhere in a 6-volt starting system should have over 1/10 volt drop, and the ground side voltage drop should be almost zero at every connection. All it takes is one high-resistance connection to kill the cranking speed.
Jerry
