If this is an ABC engine, do NOT run it rich for break in. Run the engine on the fuel you plan to use in it, nitro wise. When you get it started, run it at full throttle briefly, ( 10 15 seconds) and lean the high speed needle to a peak then richen it a bit (rich side of peak) about 500 rpm. Shut down the engine, let it cool (5-10 minutes). Repeat the procedure running at hig speed progressively longer several times checking the high speed needle to make sure it is not running lean. After several tankfuls of fuel of this, you can begin dialling in the low speed needle. IE set the high speed needle ( it should not change much from the earlier setting), reduce the throttle slowly to a point where the engine is running at slow speed. let it run at low speed for about ten seconds, then open the throttle. If the engine sputters and stumbles before picking up speed , lean the LOW speed needle a little at a time until it picks up speed reliably. If the engine stops when you advance the throttle RICHEN the low speed needle. I have found this procedure works for me.