I wouldn't use acetone...
Or any petroleum based solvent. The immediate results may be good, but acetone and petroleum attacks the rubber sheathing, causing it not only to deteriorate, but also attract dirt again much faster. (Btw, this also applies to fenders.) Don't use any abrasive cleanser (Soft Scrub etc) either...it'll leave minute scratches that'll turn the cord into a dirt magnet.Soak it in a pail or pan of strong detergent/water solution overnight, then scrub with a soft brush and rinse very thoroughly. Btw...cleaning 'em more often requires less work and gets better results than waiting till they're totally black...which isn't good for 'em either.