I discovered an excellent method for cleaning your i850 printhead. Remove the printhead from the printer. Use a soft rubber bulb syringe and cut the tip so that it fits nicely over the port where the ink tank is situated. You may want to use two syringes, one for black and one for colors as the ports are different sizes. Then take the printhead to the kitchen sink and begin thoroughly flushing soapy warm water using a little dish washing soap like Dawn or Joy and flushs the water back and forth from both the printhead slits and from the top screened supply side. Do this as long as you like, but do not quit until the whistle clean water flows freely from the supply side and exits the printhead
slots. There should be absolutely no color left anywhere...black or color. Don't worry about getting entire printhead wet. It may submersed and shaken around in the water. Then shake it dry, blow it off with a hair drier for a few minutes and place in a dry place on a shelf and let it sit for at least overnight.
Replace everything and you should have perfect printing.
It sure worked for me when the two year old printer just completely stopped printing. I think the black ink has something in the mixture that dries up and clogs worse than the colors. Good luck.