Lexmark 640 printer, will only print in yellow . Can I soak the black print cartridge in hot water for a few minutes. It has not been used in a while.

Try it.......

Remove the print head and soak it for three 5-minute cycles in hot (~170 F) distilled water about 3/4 inch deep with continuous mild agitation. The first soak quickly turn the water black, the second end with the water a dark grey color. The hot distilled water dissolved the dried ink in the small ink passages from the cartridges to the ink ejection part of the print head.
Carefully dry the print head as per instructions, clean the electrical contacts on the printer, and placed the print head and the ink cartridges in the printer.Turn off the printer, then unplugged it for one minute.Then turn it on...and print an alignment sheet...

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The following may work: remove all the ink supplies. Remove the printhead and soak it for about 5 minutes in 0.5" of very hot water. Gently blot the printhead on a soft damp paper towel to remove the excess water, then replace in the printer and then reinstall the ink supplies. get more help to fix printer not printing black hope this helps you.

Thanks
Michel

unplug the power cord from the outlet and wait 3 or 4 minutes.
Plug it back in and choose the alignment program.
a sheet of paper will print out a series of lines and etc.
once finished , the printer cartridge is alligned properly and will print nice and clear.

The following may work: remove all the ink supplies. Remove the printhead and soak it for about 5 minutes in 0.5" of very hot water. Gently blot the printhead on a soft damp paper towel to remove the excess water, then replace in the printer and then reinstall the ink supplies.

Hello
Yes you can repair by trying the following steps:-
Remove the cartridge. For models where the print head is in the printer, drop seven to 10 drops of isopropyl alcohol (rubbing alcohol) down the ink receptacle area. This is the hole where the ink flows into the head from the cartridge. Run a few self-cleaning utilities. If necessary, allow to sit a few hours or overnight and repeat the self-cleaning utility.
Soak the cartridge print head in hot water if the print head is in the cartridge. Allow to soak overnight. Dry the print head area with a paper towel. Repeat the self-cleaning utility.
Place the cartridge print head in isopropyl alcohol if the hot water soak did not unclog the print head. Leave the print head covered in alcohol overnight. Use a damp paper towel to wipe off the print head area. Dry carefully. Try the self-cleaning utility again.
Hope it helps, if so do rate the solution
good luck

If you haven't used the black print option, as in to print without using the color cartridges, then the black print head may be 'gummed up'.
You can try to clean the print head using the 'clean' function, which may work.
You can also try to get the print head to work by using ammonia and hot water. If the cartridge has its own print head, you can use some hot water on a paper towel and set the print head onto the soaked towel for a few minutes. After it has sat for a while, gently dab it dry with another paper towel, and try to use it. If it works, you're done. If not, you can use ammonia to try to clean the print head using the same tactics as using water. Try water first, then ammonia.
If the print head is fixed in place, you have to remove the whole assembly from the printer, and then try to clean the print head using the same method.
tom

If these are new cartridges, make sure you removed all of the protective plastic wrap that comes when packaged.  usually its red , yellow or orange plastic tab that should be removed.
If these are not new cartridges, you may need to try another set. 
Try running the head cleaning utility under the printer utilities options HP is very good at having this utility available to trouble shoot most common problems.  If you don't have this utility, try updating your printer driver. (basically uninstall the printer and download the driver from the HP website, and reinstall the printer using the latest driver).
Hope this helps..
~enov8

Unfortunately the printer wont print unless there is ink in both cartridges

The "Print Head" needs cleaning or replacement. It's the part that delivers the ink from the cartridge tank to the paper itself.Mostly caused by air entering the lines and making any residual ink cure.
Most newer printers only ever use 95% of the available ink before reporting tank is empty,and will not allow any printing to resume, preventing air being introduced to empty lines in the head.



Without Removing Anything

There's commercial products available, I guess a bit like using a head cleaning cassette in your VCR - I Have not tried any of these yet.

Use an empty ink cartridge Refilled with hot water and run the print head cleaning function.
( Dont worry , The paper will absorb the minute amount of water being dispersed.)

If that fails you should remove the print head cartridge most are pretty easy ,
LexMark X1150 X1185 and other similar models with the sensor strip running through the cartridge instructions are located at the following FixYA topic
http://www.fixya.com/support/t181337-piece_fallen_out_printer

Immerse it in warm-to hot water for a while or try using rubbing alcohol or similar Non-solvent based liquid.
Inspect the small rectangle shaped metal print head & surrounding copper track for dried ink , remove any with very gentle wiping taking care not to damage anything.


If it's beyond cleaning it will have to be replaced .Try ebay or the local print shop.

Remove cartridge and remove the mains from the wall.
Try cleaning the contacts with a cloth dampened WITH WATER, wipe it up with another cloth and let dry.
If your old cartridge is not working again, try with a new one, generally is the cartridge which may become damaged, not the printer... hope !