More good suggestions keep coming, I see. Thank you all for your input! Here's what I have done so far: .-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-
I bought a bottle of Redex. With a warm engine i Squirted 1/4 of a bottle slowly into each carb at medium revs. Then added a little of the stuff to the tank, put the air intake tubes from the air filter back in place and took the bike for a good spin with several full throttle accelerations. I really heated it up, both the engine and the brakes. .-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-
The result: Well, the engine feels like it is running somewhat better and smoother now. However, I can't say of this is because of the Redex treatment or simply because of getting the mixture right and/or running the bike like i did, which is harder than normal, I can say. But at a cost of 4 litres of petrol, it doesn't ruin my personal economy. One big disappointment must be mentioned, though: It didn't smoke! I was looking forward to the biggest cloud of smoke since I put some "snake oil" in my Sachs 2 stroke, but this stuff produced very little, if any, smoke at all.

At least it didn't do any harm. .-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-
Conclusion? Hard to say. I guess that's exactly what the Redex manufacturer want. It may or may not give an improvement, but one way or the other it is impossible to prove anything. The cost is also small, so one doesn't bother to complain even if the improvement isn't easy to . .-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-
Still, I may even try the rest of the bottle in my Model G. See if that does any good. .-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-
Regards, Anders F. R.