On my TT .39 I'm reading as much as 40 or 50 degrees different from one side of the cylinder to the other. The side with the muffler is the hottest. Which one should I base my engine temp on to set the needles?

Thanks

May be the plug should be the best reading point but imo the most important thing is to take the reading always from the same point. Donīt worry about the main temperature but the diferences of temperature you can read from the same point after and before any pitch or needle adjustment. i.e.If get hotter readings your using too much pitch or your mix is too lean.
Elossam

Well I'm breaking the engine in, so I'm gradually trying to lean it out. Not sure how to tell when to stop leaning, though.

I've seen people say ~240F is the hottest it should be... the side with the exhaust is reading about 230, but the other side of the cylinder is reading 170-200.