Sounds like a bad regulator / rectifier. CDI part number is 197-6G5-A0. You should read 12.5-13 volts with the motor running. The tach also gets it's signal from the RR. CDI Electronics has a troubleshooting guide if you look up the part.
1. At 800-1000 RPM, check output on the gray wire, reading should be at least 8 volts with a DVA meter. A low reading usually indicates a bad regulator if the system is charging the battery. 2. Check the resistance between the gray wire and engine ground. You should read above 100K (100,000) ohms. Gray to red, and gray to the yellow wires should be a high reading, usually in the M range. Maximum Output Test 1. Install an ammeter capable of reading at least 40 amperes in-line on the red wire connected to the starter solenoid. 2. Connect a load bank to the battery. 3. In the water or on a Dynometer, start the engine and bring the RPM up to approximately 4500 in gear. 4. Turn on the load bank switches to increase the battery load to equal 40 Amps. 5. Check the ammeter, 16 Amp systems should show approximately 16 Amps and 40 Amp systems will show approximately 20 amps on each regulator/rectifier. 6. If the amperage is low, A) Check the load bank for battery draw. B) Reconnect the ammeter between the red wires from one of the regulator/rectifiers and the terminal strip. Retest. You should show about 20 Amps from each regulator/rectifier. C) If the output is still low, check and clean all connections between the battery and the regulator/rectifier plate. 7. If the amperage is correct, but the battery voltage remains low, replace the battery. Bench Test Diode plate check: Test the forward diodes between the two yellow wires and the red wire. You should get a reading of about 45K (45,000) on one and a high reading on the other. Check the resistance from each of the yellow wires to case ground, you should get a reading of about 56K (56,000) on one and a high reading on the other. The red wire should read about 14K (14,000) ohms to ground. Tachometer Circuit: Check the resistance between the gray wire and engine ground. You should read above 100K (100,000) ohms. Gray to red, and gray to the yellow wires should be a high reading, usually in the M range