Here's a quick way to test the battery's cranking power. Without turning the ignition switch on, crank the engine for 15 seconds with a voltmeter attached directly to the battery posts (not the cable terminals). At the end of 15 seconds, while still cranking, the voltage should not be below 4.5 volts.
Turn the switch on, crank up the engine, and immediately go to a fast idle (1500-2000 RPM). The voltage should rise to 6.5 volts or more, and the dash ammeter should show at least 1/2 scale deflection toward the Charge or + side. Run the engine above idle speed for 10 minutes or more. The ammeter deflection should gradually diminish and the voltage should rise to 7.5 or so. If the system passes all these tests, you're good to go.
The most common cause of slow cranking and/or hard starting is loose and/or dirty cable connections or too-small cables. 6 volt systems need lots of current flow. #0 cables or larger with VERY clean connections and a good ground wire between the engine and the frame will go a long way toward quick starting and good cranking speed. There should be no more than 2/10 volt loss across any single cable connection, and NO voltage drop from one end of a cable to the other.
Jerry
