I have a question regarding voltage regulator of B&S engine. I'm using in my mini jeep go kart a 12.5HP B&S engine from old Craftsman riding mower originally without voltage regulator. One of the wires that comes from the stator have a diode on it to produce DC current. I don't know if the voltage regulator that I bought have a built in rectifier or not. The voltage regulator comes with two yellow wires as inputs and one red output. Those yellow wires makes me think that the input to the regulator will be AC current and not DC but I don't know.
I take the measurements of the circuit with battery connected and reads 12.4V and disconnected reads from 6V at idle and when press the throttle increasing to about 18V. I think that the voltage will be about 14V at high speed and a little lower at idle.
Take a look at this manual, starting in section 2 page 38. It tells you about the different charging systems and how to test each. You can't mix different systems with different parts. You must match the stator and/or regulator.
Your engine has a very basic 3Amp non regulated charging system. The Regulator you have is for a 10Amp regulated system. There is a big difference in the flywheels and stators between the two systems. As Rivets mentioned you can not mix and match. The only way to increase your charge rate to 10Amps with the regulator you have purchased is to also replace the flywheel and stator.