Collect all the necessary supplies - cotton but, rubbing alcohol, and the dirty cartridge (s).
Dip one of the fuzzy cotton ends of a cotton bud into the rubbing alcohol.
Slowly rub the wet end of cotton bud on the contacts of the game cartridge. Slide it back and forth touching every receptor.
Flip the cotton bud over to the dry end then soak up the rubbing alcohol on the contacts of the cartridge.
Keep repeating the first four steps until the cotton bud comes out clean. Cleaning Cartridges If the NES still doesn't work, the contacts inside the machine may be dirty.
This method could be used to clean most other cartridges aswell
How to clean your nes console
Take off the six screws on the bottom of your NES.
Remove the lid so you can see the "guts" of your Nintendo.
Remove the metal shield protecting the innards.
You will now be able to see the black part where the cartridge slides into the machine. At the back of it you will see the gold coloured receivers for the cartridge. Get a soft bristled toothbrush, that you don't care about, and dip it in rubbing alcohol.
Slowly poke the bristles into the slots where the cartridge is inserted and carefully scrub.
Reassemble your NES and let the contacts dry off before you use the system