There's a quick and dirty battery capacity check you can do without a load tester. Disable the ignition by taking the wire(s) off the coil + terminal so the plugs won't fire. Then crank the engine for 15 seconds and check the battery terminal voltage while the starter is still running. If necessary, have a helper do the cranking while you measure the voltage at the battery posts. After the 15-second starter run the terminal voltage should not drop below 9.5 volts. I'm going to guess the battery will run out of steam long before 15 seconds is up. If the cranking speed slows dramatically, stop cranking- - - -a very slow starter speed cuts down on the inductive reactance in the starter windings, and you can roast the starter from overcurrent if you let it grind slowly.
If it passes the cold cranking test, try the same procedure when the engine is hot. There shouldn't be a noticeable difference in performance.
Jerry
