Voltage meter jumps around a lot but when I rev slightly it moves to charging. I can borrow a 6 volt this weekend and try that. As someone earlier said, I don't think I can trust that PO properly installed this new battery.
Shouldn't that be an ammeter? Unless someone changed the gauges. The back of the gauge has no connections (see picture) rather it picks up on the magnetic field created when the circuit is on. The front of the gauge should say battery. with charge and discharge markings.
What do the rest of the gauges do when the engine is running?