----- And that's the problem, because at this moment the arm "A" bends a little in sense of green arrow and later it works wrong. When I found this, I've bent arm "A" in oposite sense and my gearing was perfect again - wow! I immediatelly went to our garage and bent arm "A" also on 1,6 and believe it or not, from that moment it gears perfectly!
It's very simple but maybe this is a gear problem on your Murenas
I will probably try your solution to the problem. As I understand, you straighten out arm "A" on the picture, so that it is more of a right angle with the tube "B". This will make the end of "A" move further, since its length with respect to the pivot point becomes longer.
I have tried to see if the links (K, D & F) have been put in the wrong places, because there is not enough thread on one or two of them to adjust their length, so that "A" is straight up, and "J" straight forward, when in neutral. Swapping them around does not seem to help, so I am interested if anyone knows what length they should have when they are perfectly adjusted?
Update.
While analysing how the links work and trying to discover what is wrong, I discovered that the pivot point ("I" on Anders' drawing) may be the wrong type. It is not a ball and snap-on socket, like the others and as shown on the drawing, but a ring with an imbedded ball with a hole through.
This has a limited range of motion, since the ring hits the bolt or the bracket when twisted. I put some shims under it, so it could not touch the bracket when moving the gear shifter side to side. But it may not be enough. I will have to get an original type and see if that helps.
Another possibility is that the bend in the tube "B" is too big, causing the the pivot axis to be through the center of arm "A". If this bend is straightened out some, the axis will move to the bottom of arm "A", thereby increasing the range of movement at its tip. There is 6 cm clearance under the engine to the tube, so it can be straightened out some, though not perfectly straight.