since you have the exhuast off open each valve a give a real good squirt of possum piss down each cyl. better yet get a large bore 60ml catheter syringe (most farm supplies will have them and your local vet will) and some diesel n oil mix and give each one a good dose say 2 syringe fulls and let it soak it might free it up. you should still pull the heads though to clean it properly.