Fermented nectar can make hummingbirds sick. The same goes for honey. Don't use a honey mixture in your feeders, it can grow bacteria fatal to hummingbirds. Cleaning your feeders is very important, and should be done every time you add fresh nectar. Again, I've had conflicting reports on how to clean feeders.
Some experts recommend cleaning with soap and water, or with a mild bleach solution.
Others warn against using soap or bleach, which may leave a residue that harms the birds. If you don't have a brush, and you can see residue inside your feeders that you can't reach, try putting a tablespoon of dry, uncooked rice in your feeder with some hot water and shake thoroughly. Rice is abrasive enough that it should scrub your feeder clean. Strain out the rice and throw away, and rinse your feeder thoroughly. Cotton-tipped swabs or an old toothbrush can be used to clean feeder ports.