First, if you don't have a good voltmeter, get one. For general troubleshooting, a simple digital multimeter will do. If you're planning to get into adjusting a generator/regulator system, an analog meter with a sweep needle instead of a digital readout is better.
Check the voltage at the battery posts (not the cable terminals)with the ignition off. It should be slightly above 6 volts, maximum 6.3. Next, with the ignition off, crank the engine for 15 seconds and measure battery post voltage while still cranking. It should not drop below 4.5 volts for this test. If the battery fails either voltage test, charge it fully and retest. Batteries can be bad right off the shelf- - - -"new" is a relative term, especially for 6V batteries that might sit on the shelf for months before they're purchased.
Next, measure the voltage at the big starter cable while cranking. It should be within 1/2 volt or so of the voltage at the battery. If not, you have dirty connections and/or too-small cables. 6 volt systems simply will not tolerate voltage drops. All connections need to be clean, no make that CLEAN and TIGHT and as big a cable size as you can find!
Next, check the voltage at the coil + terminal while cranking- - - -it should also be within a few tenths of a volt of battery voltage at the posts. If not, trace the wiring back through the ignition switch, ammeter, and ammeter power connection at the starter, looking for loose or dirty connections and/or voltage loss through the ignition switch.
Finally, check the points in the distributor- - - -they need to be clean and gapped properly, and the ignition timing needs to be adjusted correctly.
You're probably going to find battery problems and/or dirty cable connections. Good luck!
Jerry
