So here is the drill...............

Use a low nitro fuel that has castor as one of the oils. You can also add a couple of ounces of castor to any gallon of fuel and it'll work.

Crank the engine and immediately bring it to a 4cycle. Then turn the needle in until it just breaks into a 2 cycle.
Lean it out until it starts to slow down. Immediately when you hear it start to slow, richen the needle until it picks back up.
You've just discovered the needle setting near where the piston/sleeve will run safely and break in. Give it a couple of clicks rich for insurance and start the run/cool cycle.

After it is cooled down, crank it and let it run a couple of minutes, and shut it down and let it cool.
Repeat that a couple of tanks.
Test the needle setting every once in a while to see if the "high speed sag" needle setting has changed. If it has and it's at a leaner needle setting, you still have some breakin to go.