This engine has an ignition unit with an advance curve that will cause the idle to hang around 2000rpm for a few seconds. You will need to adjust the low idle so that you have a slight burble at low speeds to help it come down. Check also for air leaking and if you have not done so remove the idle stop screw completely but leave the throttle spring attached. Make sure the butterfly is exactly centered, it should not be sticky. Check the DLE 20 thread also for more solutions