Good advice above!
Question to OP:
What makes you think your bike is running too lean? What symptoms or problems are you having? Please describe.
Remember: The DR650, with stock BST 40 Carb, comes very lean from the factory ... and it runs just fine like that. If you're having a specific problem, please try to describe it so the DR650 crew here can try to help out.
As Gipper mention, the guys over on DRRiders forum also very good and helpful. It's all there, or .. just ask!
Also, Gipper's suggestion of the extended fuel screw is a "MUST DO" item, IMO. You will need it crossing the Rockies.
I wonder what the current jetting set up is on your bike?
Has the Carb, Air Box, air filter, Pipe been modified in any way from stock?
If you can detail out your set up (if you know), I may be able to "solve" your problem. You may be running RICH, not lean! (this has happened before!)
So, What Is:
Jetting? Main jet size? Pilot Jet?
Air Box modified or Stock set up with snorkel or aftermarket air filter or Cut Air box? If you don't know, just say so.
If your Carb has a kit from Pro Cycle already installed (or a Dyno Jet kit or other) then what needle do you have and what clip position are you using?
All this matters and is mostly easily remedied by owner in Motel parking lot in a few hours of work. Or ... visit a good shop but expect to pay pretty high hourly rates these days.