This is the case:
-If I open the throttle by ~ 10%... its reach 2000 RPM then its drop to 1500 RPM then weak up again to 2000 RPM and so on.
-If I open the throttle by ~ 20%... its reach 3000 RPM and stay smooth.
-If I open the throttle by ~ 100%... its reach ~ 4200 RPM then its drop to 4000...