I have the 150 irvine and this is what the manual says, 120 will be the same.

"With a ringed engine the idea is to run it VERY rich for the first few tankfulls punctuated by small bursts of leaner running to allow speeds to build up for short periods. This should be carried out at FULL throttle using the main jet needle to control speed changes. After the first tankfull the model can be flown."

"As more running is achieved the leaner periods can be prolonged until about one hours running has been accomplished. The engine should then hold full power without fading. If the engine shows signs of slowing, richen the main needle mixture immediately."

You want to use a prop with less engine loading for break in, like a 15x10 or 16x8. The main needle should be 3 turns out for the first few tanks. Hope this helps and good luck with your engine.

BTW these big 2 strokes don't like high nitro fuel. The manual says 80/20 with 5% nitro.