Since you mention that you bought it used not sure if its a old old model....while the fuel line replacement was a good idea did you open the fuel tank and replace that line....look at the clunk if its a brass one it might be corroded from the nitro....

running engine upside down is no different the upright......you can still flood an upright engine and lock it! If you really have problems my suggestion would be to mount the engine on a test stand and adjusted since you seem to be a new flyer, that make thing just a lot easier.