Try taking the petcock off the tank and make sure the intake screen is clean.
While it's off, take it apart and make sure it's clean inside , too.
If it were mine, I'd pull the carbs off. They'll come off all together, turn them upside down, pull the float bowls, and measure the height of the floats.
One or more of them could be set too low. If you don't have a manual, do a search for the specs online. While the carbs are off the bike , pull the tops off and check the diaphragms for any tears or pin holes. At that point, you might as well pull the main jets and inspect them too.
While the main jets are out, check the number on them and compare with known oem numbers to make sure you don't have a botched rejetting job
done by a PO. I've found Ron Ayers to be a good source for on line mircofiche
to check oem numbers.
How about the tank vent? Is that working properly? Do a WFO throttle run, the pull over asap and open the fuel cap. If it sounds like it's sucking air or is slightly stuck when you open it, it means the tank is not venting the way it should.
You really should buy an aftermarket shop manual. They're cheap on ebay or amazon. I paid $25.00 (shipped) recently for a DRZ manual, as an example.
Worth it's weight in gold when you've got your bike torn apart. Plus, it'll have a good troubleshooting section.