Take a covering heat gun and heat the engine up real hot then just turn the prop. If you don't have a heat gun the the kitchen oven will work too. Castor oil gums up after time and the heat will soften it up. I then break down the engine and clean it up using a tub of boiling water and rags then use after run or Marvel Mystery Oil and reassemble the engine. If you have a lot of engines that need cleaning up then I use an {OLD} crock pot and anti freeze to clean them up. Not worth the time or trouble for only a couple of engines though.