Hi Patrick.
Your English is pretty good but I think some of us may need some things clarified before we can help.
Firstly, do not be scared of the engine having fuel injection. Apart from the electronics the basic principles are similar to carburetors.
A 'cracked inlet pipe' can actually mean the main engine breather pipe. Its common on older K bikes for the rubber to crack with age and can cause the problem you describe keeping the engine running.
Examine it carefully for cracks and renew it if necessary.
Can you explain what you mean by 'glow discharges'?
Do you mean electrical sparks or burning fuel ?
If its burning fuel, check the O rings on the injectors and make sure that they are not cracked or maybe not even there! Sounds like fuel might be blowing back from the injectors past the O rings.
I doubt if its the fuel regulator (but not impossible, of course). 
I'm going to say it before someone else does. Many electronic issues are caused by a build up of corrosion in the multi pin plugs on the boxes under the tank. Always a good idea to go over them with a good quality contact cleaner when you get a chance. Most on the forum would recommend 'Deoxit'.
The main thing to remember when working on the K is to start with the simple (cheap) things.
These engines are bullet proof but are getting older now. Its best to start with things that may be affected by age and corrosion first.
Don't worry, its very unlikely to be serious and the guys on the forum love a challenge.
Keep giving us as much information as possible. And keep a fire extinguisher handy!

I have seen a K100 backfire hard enough in the inlet to blow the injectors out of the hole and then you will see some fireworks in the ports.

Dear Comberjohn and Dear RicK G,

thanks for your quick responses.

Since you are talking about welcoming challenges, I'm going to add some more and detailed informations.

What I worry about injection are all those devices managing sparking timing and fuel/air quantity. i.e. a failing fuel pressure regulator, or its rubber hose, should increase the amount of fuel in comparison to air. Please consider that I'm a former 1977 Moto Morini 3 1/2, 1973 Laverda 750, 1972 Moto Guzzi 850 GT, 1971 Moto Guzzi 850 California and 1982 Moto Guzzi 1000 SP DIY owner. Not easy to repair, but easy to understand.

Despite the three or four years under rain and snow, the K75 conditions are pretty good. I'll post some pics ASAP. This means that air filter, gaskets, rubber pipes are all quiet good. In addition, no vandalism at all, during these years.

Fuel in tank is new. Spark plugs are new. The oval gasket of the air intake to the air box seems to be replaced this morning! Contact surface between the two halfs of the air cleaner box (do you call it breather?) has been greased by me smoothly ensuring a better sealing action. Fuel pipe from tank to injector's rail has been replaced by me. Oil has been changed, but before driving the bike on street, I plan to run the engine warm and provide a second oil change and a new oil filter will be fitted in. Synthetic oil is able to "clean" and "loose" old and sticky one. Multi contact plug under the seat (L-Jetronic) and a lot of other contacts have been sprayed with de-oxydating product.

Now, glow descharges are light flashes... burning/exploding fuel. Fuel-air mixture igniting and lighting out and up the fire chamber.
Therefor I guessed it could be a stucking intake valve. But all clearances are correct. Blowing compressed air in the spark plug hole with closed valves moves the engine, rotating 5-6 degrees, until compression becomes too high. I suppose an open valve wouldn't allow this.

Glow descharges mean also bangs. Coming from the same area.

Not willing to desassemble air breather, injectors' rail ... etc. befor having some more infos, I tried to check all rubber parts touching and looking for cracked surfaces. They are all good, so far I'm able to feel and see. The only gaskets I don't like are the three couples of O-rings of the injectors. A little bit too dry/hard. I'll replace them ASAP, but they are actually not so bad to give me head aches.

A german K-rider suggested to replace all rubber parts around the injector rail. Ok, it's not a matter of money, but of time wasting (here in Italy it's not so easy to find these spare parts). But I will not turn somersaults if an intake stub cracks again because of back firing! 

Nevertheless, I've a question mark in my mind (is it correct to say so? lol). What are we dealing with? Rich or poor air/fuel mixture? And how is it possible that the ignited mixture finds a way back out of the firing chamber? Or did the banging fuel never enter the firing chamber? This behaviour should mean exceeding fuel!

Should I focus on rubber or contact oxydation?

Thanks for reading

Patrick

I would start with checking the ignition timing and valve timing. the thing to be wary of with a K75 is that the timing is done with TDC on No3 cylinder not No 1 cylinder as it is for the K100 and almost every other motorcycle and car in existance.
Check also the firing order is correct
The incident I refered to was because the plug leads were reversed on 3 &4 cylinders so there was an huge bang and it even blew the injectors back out of the head
As you say a stuck valve is not likely as the clearances are correct. Having been in the motorcycle trade for a number of years a customer or the seller will tell you all sorts of rubbish as to problems rather than admit they did something wrong and now the bike wont run.

Rick G,

I descovered that the vent hose from engine to air filter is fully cracked. Inserting a rubber plug into the filter hole seems to provide a better result, but it still bangs and flashes. After following your tips (I need to find my strobe timing light or borrow one), I'll desassemble everything above the engine and replace all rubber parts, anyway (intake manifolds, injectors'  O-rings etc.)!

Hi Patrick,
Great to have you with us, I strongly suggest you shift your troubleshooting over into the technical part of the forum (Technical Repair & Troubleshooting) as not all of the gurus read the New Members section and you will miss out on their input.
Regards,
K-BIKE