Sun MDT distributor tester

Ok I hooked up a nearly dead 6 volt battery and the gauge lights all light. The vacuum pump seems to be trying. The dwell meter is inop. I do not have a square tool to tighten the dist. gear so I am assuming that is why the RPM is bouncing a bit and the arrows with the strobe are walking around the circle. The volt gauge is reading 6 volts but I did not put a digital volt meter on the battery.