As long as it is soft carbon you can use a bottle brush made for cleaning oil galleys. Might be able to find them at Napa or other large parts store. Wash the stuff down that can not be removed with a rag from the top. CAUTION-USE CLEANER THAT EVAPORATES AND SPIN THE ENGINE OVER WITH THE INJECTORS OUT TO DRY OUT THE CYLINDERS TO AVOID A RUN AWAY. If it is hard carbon you probably don't want too much getting into the cylinders so best to leave it alone. Make sure the copper sealing washers all came out with the injectors.