On my K100, I have a separate voltmeter which shows a healthy 13V up to 60MPH or 4,000 RPM in 5th gear.
However, as the revs and speed rise further, the voltmeter drops down to 11V and even 9V by 80 MPH.
At no stage does the red alternator charging light come on on the instrument panel.
The battery is new.
When the speed and revs reduce below 4,000 RPM, the voltmater returns to show 13V.
Is this normal or perhaps symptom of an alternator problem ?

13v dont sound that 'healthy' to me Pedro
it looks a wee bit on the low side as even 13.8V is about trickle charge/float voltage
My K regulates nearer 14V.
maybe you need new brushes?
I'd test it against a known good meter on the battery terminals first though as some of the shop bought ones
can be vague at best and depending on where and how your meter is connected,
this can affect the voltage.

Thanks reg_K100RS.
I will put a Fluke metre across the battery and see what the readings look like as the motor revs increase.

Pedro
Just been tinkering outside and put my (Fluke) meter on the battery terminals
Low voltage initially probably due to the battery being a wee bit flat.
Shortly after when the battery has recovered a bit and I run it at say from 1500 - 2000 rpm onwards
I get 13.9V near as dammit and thats with headlights on or off.
Not much more than battery voltage at tickover prob due to low alternator speed
HTH
Reg

My guess is dodgy worn out brushes on the alternator regulator, You can get just the brushes and change them or get a new regulator with new brushes. That will fix your problem I believe.
Regards,
K-BIKE

Thanks for the advice. In fact measuring across the battery with the Fluke meter shows a steady 14V revving up to 4000 rpm. At the same time, the bike´s voltmeter drops down to 11V so I guess reg_K100RS was right and the problem is the voltmeter itself- So that´s alright them.

Many thanks