A tip that I've been using for a few years now for cleaning carburettors and related engine parts is to clean with cellulose thinners-use a large bowl and a small brush, use in a well ventilated area or outside, and it gets all the gum,dirt and residue off relatively easily and leaves the aluminium ready for polishing or painting.
I did the carb and distributor this way, and lightly polished it with metal polish to keep any corrosion away (I'm repainting the distributor advance),lightly oil any moving parts, should look good when the engines repainted and back in!

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[/frame] You can get 5 litres of basic thinners of eBay for around £10 with free delivery and it lasts for ages.