Hi Arnko37,
Several problems can cause your motor to die below 2000 RPMs. Don't discount the possibility of a stopped up carburetor. The motors are test fired and often the remaining fuel in the system will dry up and leave residue in the carburetor. Another problem to consider is the paint chips from where the gas cap is attached to the tank, the chip can easily pass through the filter and stop up the carburetor passages. Also look to see if the head is leaking compression near the center rear head bolt. Because of the latest design the head doesn't always seal correctly. Of the last 8 or 9 motors sent to me for repair "all" of them had the camshaft advanced 2 teeth instead of 1 and caused the motor to idle very poorly.
On a few motors the intake sleeve didn't cover the hole caused by the rear center headbolt and leaked into the intake port.
Keep us posted on the progress. You can contact me at 252-475-0406 for additional information and possible fixes for your motor.
Have fun,