I am a UK citizen working on a J1 Scholar Visa in the US. I am currently under the UK-US Tax Treaty to be exempt from taxes as long as do not stay longer than 2 years in the USA. I would like to know whether this Exemption rule would be invalidated if, after having left the USA, I then returned to the USA on a tourist visa, on holiday? I/e/ would re-entering as a tourist influence the 'retroactive loss' rule of over-staying the 2 years? I think not but any advise or experience greatly appreciated!

Many thanks

You have nothing to worry about if you return on a tourist visa and make no attempt to earn any income while in the United States.

You would only have problems if you returned on a work visa, like E-1 or H-1 or H-1B.