I have a Mini 1000 that has sat for 20+ years, as such the engine coolant had dried out. There is a caked on mess at the bottom of the water jackets. I tried to unblock the engine drain using a screwdriver and compressed air with no luck. I removed the water pump and was able to scrape some of the gunk out, but there is only enough room to get around the first cylinder. My question is what would be the best way to clean out the rest of the water jacket?

 

try vinegar. pour a bottle in and let it sit for a coupla days There are other solutions that are stronger (muriatic acid for one) but they are quite caustic and can cause burns, eye irratation, etc. Vinegar works well at dissolving deposits, and is generally quite harmless