Hi,
The best way is to disconnect the cable housing at the tachometer and at the engine. Then pull out the cable for wear and rubbing.
The housing can be cleaned by putting a flexible hose over one end or the other and forcing stoddard solvent down it. You can spray the solvent from a portable garden sprayer. Catch the crud as it comes out the other end by putting the other end down in a gallon plastic container. Then blow it out with compressed air.
Back when cars had mechanical speedometers I used to get a special cable lube from the local Oldsmobile dealer. I just typed tachometer cable lube into my search engine and came up with http://www.summitracing.com. It has Dorman Graphite cable lubricant for $3.99 for a half ounce. I would probably get a couple.
Another option is to get a tube of Aeroshell #7 from Aircraft Spruce, or the aircraft parts supplier you prefer, It’s a low temp multi purpose grease recommended for instruments. Good for temps ranging from -100 def F to + 300 deg F.
Lube the cable after the housing has dried, slide the cable back in, making sure it’s indexed properly to the tach and engine and you should be good to go for a long time.
