Just replaced the engine in 748RS, upon starting everything is fine but once warmed the dash gives 'lo battery' signal. Checked battery (which was flat on start up) and the voltage is low. Idling goes up and down with volts. I suspect faulty rectifier or generator but don't know the figures for checking them. Seems strange the rectifier would go just as motor is changed which made me think of the generator but the engine was supplied as 'checked' only 212K from monster mob. I have a couple of other generators but all give different (ohm) readings across the output pins!?! Checked connections and all seem fine but will go over them again to be sure. Any ideas on testing the rectifier etc I dont think it is the same as the standard one.
The first thing to do is to change-out the battery (borrow one if need be) given that the battery was flat to begin with. Even a good regulator won't be able to charge a bad battery.
Matt, try the connections on your generator,sometimes a bad connection can cause this and also make sure you have the air flow meter connected this can be over seen when you change your engine and also causes this to happen,let us know how you get on.