Do you know what muck is in it and where? If its furred up like a kettle then central heating descaler works, this can also dissolve iron deposits, you can run with it in for a few weeks then power flush by connecting a garden hose to one of the engine connections (with a home made adaptor). If its dirt in the block you may have to take the core plugs out and jet wash it then put new plugs in, they are very cheap. Fernox gave me some good advice when I had a Merc with a furred up alloy head, they said they had worked with the RR owners club on descaling older Rolls with alloy heads. The indutrial powerflush heating cleaners are seriously strong acids, (phosphorcic I beleve) and will clear most things, but you need to get it all out and you should probably neutralise it with an alkali flush too. Not a sucess if the system is clean but the heater falls apart a month later!