Voltage 'surging'

[bobber01]

My '80 CBX has a charging issue which is frustrating me. When I start the bike, it shows a little over 13 volts then, once rpm's increase, the voltage 'bounces' between 10 - 13 before finally settling at 10. Ride the bike for about 5 mins then the 'bouncing' returns with the voltage settling at 13. After that , no issue. Volt meter shows normal operation (12-13 at idle then 13+ once off idle... remaining steady). Can turn bike off, go for a Timmies, come back and voltage works way it should. However let bike sit for a couple off days and issue returns.
I didn't realize just how much of a parasite the alternator is until hearing the engine when the voltage 'bounces'. I've replaced the battery, brushes and alt plates. The alt spring seems strong and there's 2 washers behind it. I had put in Motul Double ester oil at almost $20 a litre and went back to syn oil thinking it may of been the oil. Any ideas???

[bobber01]

For anyone interested turned out to be the regulator. Tested the stator, rotor and regulator to find, while stator, rotor at the max end of specs, the regulator was out. Replaced with an after-market (as OEM no longer available) and bike charging as it was before.
Still cann't figure out why it was acting the way it was. Odd watching the voltmeter 'bounce' but sure was loading up the engine when charging! Must of been a weak 'switch' inside measuring battery voltage and sending a charge to rotor. Since the FJR is down for a couple of weeks apparently waiting for a fork seal I'll be back on the CBX for awhile.

[EMS]

Thanks for the Follw-Up! Good info. How did you handle the wiring with the aftermarket unit? I see new original regulators on ebay all the time

[steve murdoch icoa #5322]

Glad to see you got it sorted out, Bobber.
Any idea why an extended down time would affect it? To my mind the regulator would be bad at all times?

[oilheadron]

My charging needle bounces too, but the actual output measured at the battery is what it should be. I think mine just has an erratic gauge. I noticed that the startup bike I posted a video of last week (first-time startup after new storage for 20-something years) was doing pretty much the same thing.

[bobber01]

The aftermarket wiring had the same connectors and color-coding as OEM, but the mounting... grrr! OEM mounts flush whereas aftermarket bolt holes are recessed. Aftermarket also wider so needed to be shimmed to clear a bolt on L/H side and 'squeak' by the rear brake switch.
Only thing I can think as to why it would bounce before charging would be a weak diode/transistor inside regulator. May of needed some heat before fully closing?
I had looked at the voltmeter thinking it may of been at fault but my own meter getting same readings so ruled that out.