Guzzi2Go

That means the regulator will shut off 0,1V later and the voltage on the battery will be 0,1V higher.

With "special failure case" I meant the one where regulator partially fails. so the lamp still works, but the rest of the circuitry does not. As a test, perhaps the output diode test Kiwi_Roy suggested a long time ago - one way conductivity between yellow (alternator) and red (output) leads. Provided you're well equipped (no pun intended) and posses a regulated voltage supply in addition to diode tester (connected to red and case), you should be able to hear a beep once input tension (black) drops below 13.8V or so, according to the schematic and data Kiwi_Roy posted earlier. And if you want to include the effects of paranormal, you can do the test above with the supply connected to light switch relay (red-black) and the diode tester to red-green and battery negative (F3 fuse removed) . See if there is a difference (voltage drop) and it takes more Volts for the regulator to shut off.

"Reference" is a misnomer here. Assuming Kiwi_Roy schematic is correct, the "voltage reference" is provided by zener diodes internal to the regulator. The 12V comes from the battery via F3/30A fuse, zero or ground goes via chassis (heat sink) and directly to battery negative (black cable bolted to regulator's heat sink). Both red-green and red-black wires from the regulator are positive (12V). Blue line is the return line (negative) for the charging lamp. Once pulled low, the lamp goes on. The lamp will flicker or be permanently on whenever battery's tension is above the preset regulator's tension. On low/idle RPMs AND full battery the regulator may not achieve "reference tension", hence the flicker.

#MeToo

Of course you'd be allowed to build something with a brand new frame. The downside however is that you need to meet all the requirements valid at the date the frame is produced and that is difficult to achieve in a shed. Sometimes even factories struggle. If you take a frame from 70-ies, you need to meet noise/pollution requirements of the era and these were far less stringent than contemporary ones. Opposite example is replacing frame on a new bike (i.e. after a crash) and salvaging the rest. Some guy was selling his 2015 Aprilia V4 for small change as a racetrack only bike - frame replaced but laws changed in the meantime, thus no longer "registrable". Germany is no country for choppers, kit cars and similar. People that insist on having them must have thick skin and wallets.

Did you try Stein Dinse? They don't say they are not available, just that there is a long tead time (2-8 weeks) - https://www.stein-dinse.biz/Moto-Guzzi/Motor/Ventildeckel/-dichtungen/Ventildeckel-V11-Sport-1999-2001-grau::6662.html .

It does not. Unless you ride on ice.

Correct. It is the brake that upset the bike. The weaving or "countersteering" is just a consequence here.

So the LED lamp is rated to 25/50W, regular H4 lamp is rated 55/60W. Former costs 75$, latter ~3-4$. Is it worth it? "Heathwise", the wattage tells you what gets more hot. Obviously, regular lamps run hotter.

What is "Open Current Voltage"?

Willy or willy?

I usually go for Aral's 102 octane or 98 octane "super" when not available, hoping to reduce pinging. Not sure it actually helps. There was a big debate about which motors can digest E10 back when it was introduced, every manufacturer published a list. As I can recall, Guzzi made no statements. For my 1981 Honda it was stated that I should not use E10. So I don't use it in my bikes. Although, I am not really sure what makes Aral's 102 octane fuel a 102 octane fuel. I cheerfully use E10 in my cars. Not religious about it. Not religious about anything actually.

But that can't be right, can it? The Q3 requires positive GS of 2-4V to conduct, this will never be the case with the Z2 as shown. Also the SCRs seem to be wrong way round and unprotected. The Q2 seems to be more or less permanently on, driven by the alternator's negative half-period... Has anyone tried simulating this circuit in e.g. SPICE?

The lamp is controlled by the regulator, not ECU. So no, ECU ground is not the culprit. Flicker will be back! ;-)