Start with a known good battery. During cranking, hot or cold, the voltage at the posts of a 6V battery (NOT the battery cable clamps) should not drop below 4.5 V. If so, either recharge or replace the battery. Also during cranking, the voltage available at the ignition switch side of the coil should be the same- - - -4.5V or more. That polarity reversing switch should be disabled, unless you've got a 100 point car show vehicle than must be 100% original. It adds a bunch of potential trouble spots to the ignition system, and does not extend point life like it was designed to do. Every connection from the battery to the starter, every connection in the ignition system, and all the grounds in both systems must be surgically clean and tight. 6V systems leave no room for error whatsoever!
Jerry
