As Joe says, the engine must first be strong and properly adjusted. The solenoid must then work so that the speed does not drop when the gear is engaged.
This can be prevented by increasing the speed and CO2, but the gear changes will not be smooth. It will jerk a lot.
Is your lifting magnet connected and working?
Is the co2 set correctly?
Peter
Of course 1500 at idle is far too high.
It shouldn't go down that much when you put it in gear