Javier, regarding your 96 883/1200:

1.    Confirm 175 jet (1.75 mm)?

2.    Are you able to measure the oil temperature when the noise starts?

3.    What is the timing setting?

4.    If you remove the front (hardest working cylinder) plug, can you
look with a small light and see if a hole is starting to burn in the
piston dome?

5.    Is the idle jet stock?

6.    Is the needle stock / shimmed?

7.    Have you checked for an intake leak, which would cause a lean
condition?

I am sure other Listers will have other ideas or add to this.

Jerry