I have one of those fridge food keepers made of nylon with the snap on lid. About the size of a pound of butter. You know that last 1/2 or 1 in. of fuel that you don't use? pour it to fill to 3/4 full in the keeper and drop your engine in either together or apart. Let it soak for a couple of hrs. and use a brass wire brush on it. Wear rubber gloves and safety goggles. Nitro not nice for the eyes or skin. Will loosen tight motors and get rid of the cooked on castor oil. Keep it sealed and you can use it over and over. Really bad ones may take a couple of soakings.