Can you change a 1991 Kawasaki KH 125 from 6v to 12v? The rotor is the same part number so would it just be the four pin regulator rectifier to change. Will the points and condenser will be the same

Try flashing the rotor it has lost it's resideual magnetism through sitting.
Disconnect the rotor field wires at the AVR or on the exciter/brushes. Note they are polaroty sensitive. Connect a 12V battery to F+ and F- (note polarity) and momentarilly power from the battery for a few seconds. DO NOT forget to disconnect the rotor/exciter field wires first or the voltage regulator will be damaged.

Disconnect the rotor field wires at the AVR or on the exciter/brushes. Note they are polaroty sensitive. Connect a 12V battery to F+ and F- (note polarity) and momentarilly power from the battery for a few seconds. DO NOT forget to disconnect the rotor/exciter field wires first or the voltage regulator will be damaged.

The battery charge voltage should be about 13.5-15.5 volts dc when speed is about 2000-2500 rpm. The rectifier and/or voltage regulator could be bad or the motorcycle could have a wiring issue such as an intermittent connection/short etc.

It will be the stator or voltage regulator. You should het at least 13+ volts at the battery. The rectifier changes AC to DC.

you will need a volt meter across the battery,
it should read around 12v with the engine stopped and rise to around 13-14v with the engine running.

If the reading doesnt rise when it starts, then it isnt charging and the regulator\rectifier is the next step.

If you are getting a voltage rise and turning the lights on stops the bike, then there may be a problem in the switches, on \off high \low

Check the rectifier. Power should flow in only one direction.

Check the regulator. works?

Is the alternator putting out?

Is the battery fully charged?

Im leaning toward a shorted rectifier.

If you need help with this, please comment to this post.

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I had the same problem with my 900 that I just bought, the regulator was fried, so the local kaw shop replaced it with a new one off internet, then it promptly fried the new one, upon further investigation the rotor/armature was reading 1 ohm resistance, supposed toread 4/7 i think anyway, had to have alternater rebuilt , cost 370.00 , works fine now.

Try testing the Rectifier/regulator.

check the head switch tht could be the culprit!

First of all you must understand the power interface circuit.Mainly it has two line filter coils at very much start after the DC in socket, they usually get burnet when connected to the reverse supply rail, if you have these filter coils replace them first, if do not have, just bypass them.After these coils, rectifier IC can be found, it is rectangular in shape, has four connections, looks like an SMD IC, about 5 mm at lenght , 3 mm at width, this can simply be replaced with four diodes in bridge configuration.You can easily identify its pins configuration by tracing the tracks. The connections going to the DC socket are input and interchangeable, the other side tracks are connected to the regulation section and can be identified with an electrolytic capacitor fixed directly to the connections of rectifier.The polarity identification is directly read from the capacitors polarity.Check upto this stage & email me after testing the power interface section with the model number.