Hi, F-18.
The age of your engine also makes a difference.
If too new, the internal parts are too tight and fine-tuning is useless. At that stage, the engine needs to be broken-in, in order to achieve the proper clearance among those parts. Only after that period, the engine will hold any tune setting.
If too old, the compression may be insufficient, due to ware out piston-cylinder, or the old gaskets, carb’s barrel and o-rings may be letting extra air into the crankcase. Any of those things are able to drive any carburetor tuner crazy.
If your engine is not too new or too old, you should be able to tune it properly, using a new glow plug, new fuel, and checking the condition of the needle’s seals, the proper fuel plumbing (no air leaks nor fuel restrictions), as well as the tightness of all the bolts.
Regarding the best method, I have seen pilots using their ear, tachometers, thermometers, saliva, and even their fingers (not recommended for more than one try) for tuning engines. With some experience, you will find what works better for you.
The bottom line is obtaining a proper combustion thru a balance between fuel and air that is good for idle, transition and max rpm’s. That is a fine balance that is difficult, but not impossible to achieve, if all the engine’s parts and fuel are working properly. The jigsaw that you have to play with are the low and high needles, nothing else. That is why having the rest of the parts working with you and not against, is so important, especially when you are learning.
Some people learn by experimentation, some luckier people are taught by more experienced tuners. Don’t hesitate to ask for help, if you fly with experienced pilots. Regardless the method they use, their experience is priceless.
If you don’t have that opportunity because you fly alone, here you have some good references:
http://www.scootworks.com/rdrc/ then click on engine tuning and on glow plugs
http://www.mecoa.com/faq/carb/needle_aero.htm
http://www.rc-airplane-advisor.com/m...ne-tuning.html
http://www.flyrc.com then click on sample articles then click on tune_your_engine_1.
http://www.holdfastmac.com.au/howidlemix.html
Regards!