Cheap and not messy solvent for motor oil
In terms of inexpensive, safe, and a little polar to move oxidation products, I'd try soaking in and flushing with hot soybean oil (replace elastomer hoses and seals).  Then flush with kerosene (odorless white spirits, hardware store  paint section), then blow out with compressed air.

Carburetor cleaner was mixed cresols - dangerousy on skin (heavy gloves), but great against deposits.  If it is really nasty in there, I'd go for gun bore cleaner soaking, specifically Ed's Red without the lanolin and substituting MEK (methyl ethyl ketone) for the acetone to lower flammability. At the end, blow it out, flush with kerosene, blow it out.

Contain all wastes.  Beware of skin contact (gloves) .  Do not breath the fumes (ventilation, and maybe a charcoal latex paint mask, then discard).  Eye protection (goggles).  VAPORS ARE FLAMMABLE AND HEAVIER THAN AIR.

http://www.youtube.com/watch?v=2OfaCCJjBEU 
Ed's Red Gun Bore Cleaner  (wear gloves!)

1 part Dexron II, IIe or III ATF, GM Spec. D-20265 or later. 
1 part Kerosene - deodorized, K1 (can be Kroil or WD-40) 
1 part Aliphatic Mineral Spirits, Fed. Spec. TT-T-2981F, CAS #64741-49-9, or may substitute "Stoddard Solvent", CAS #8052-41-3, or equivalent, (aka "Varsol") 
1 part Acetone, CAS #67-64-1. 
1 lb anhydrous lanolin or  Lanolin, Modified, Topical Lubricant. 

Note that Ed's Red is rapid death to plastics, polymers, varnishes, paints, and coatings.