Just to add to Storm's advice. You will need to adjust the needle after every cut you make on the pipe, usually richer. Also, if you find that your blowing glowplugs every bench run or flight you will need to add a head shim, .004-.005 thick will usually be enough, sometimes switching to a colder plug helps too. As RPM increases, more head clearance is needed to prevent detonation. If your using a digital tach and the readings are jumping around by 200-300 rpm at full throttle, this is an indication of detonation. In my experience with some 40 size engines, going over 17k rpm requires more head clearance, and increasing nitro content also requires more head clearance to prevent detonation. The real test is in the air, you should hear a noticable increase in rpm at about 3/4 throttle as the engine comes on the pipe. Once you have a good combination of prop, nitro and pipe length, changing anything will require re-tuning the pipe.