XJ900F idling with lights on, Voltage drop.

1988 XJ900F idling at 1100 rpm.
When riding, idling at a traffic light lights off 14,V (onboard Voltmeter)
Lights on 11,2V and engine starts to stutter.

All contacts are cleaned/overhauled, normal light bulbs.
Changed generator coil 2 years ago because the original one had faulty resistance values.

What can be wrong withh my bike?

 

i would suspect the battery, it looks like it is not supplying enough voltage to keep the bike running ,when the alternator is not giving output

 

I stumbled across this the other day and per Yamaha FSM the AC Generator should supply enough current to run the bike with no discharge at just over 1000 RPM. When I did some testing a few years ago I pretty much duplicated this where at idle it took the headlight and high current brake lights to produce a negative drain on the battery.

You have a couple threads on this. I remember asking but don't remember you saying where the voltmeter is connected. If it is near the headlight connector and not connected directly to the battery then that drop would be considered normal.