Engine "dies" riding downhill

[xteiro]

Hello,
I am struggling with EC300 MY2011 of my son.
No way to keep the Engine "alive" while driving downhill at low speed.

Tried:

Main jet : 175
Check carburetor fuel level -> OK
Spark plug -> OK (grade 8)

Idle Jet 40 -> tried 38 useless
Idle Jet 40 -> tried 42 useless

thinking about give a try to Idle 35 ....