If your bike is running a Mikuni BST33 (CV carburetor) and the idle takes too long to drop back down to acceptable RPM after letting off throttle, it is likely that idle mixture and idle RPM are not properly tuned. This is a very common problem with bikes that have been sitting for a period of as little as a few months to a year or more. The following procedure should cure the "hanging idle" issue on the DR250/350 with Mikuni BST vacuum carburetors.
Prior to performing this tuning procedure, be absolutely certain that your carburetor has been completely and thoroughly cleaned. Follow the carburetor cleaning procedure if the bike has been sitting for a significant amount of time or a dirty carburetor is suspected.
The cylinder intake/carburetors boot must have no air leaks. There should be absolutely no vacuum leaks due to a worn stock vacuum petcock.
Lower idle speeds will result in less hanging. Idle speeds of 1,400-1,600 RPM are known to work well. The idea is to set the butterfly open as little as possible while maintaining a stable idle. Follow the carburetor tuning procedure to set the idle mixture and idle RPM.