ORIGINAL: Dreddi

Just a note; The short bursts of wide open throttle, then back to low power, then wide open again, is for a ringed engine. The purpose of this is to increase cylinder pressure as much as possible (which is highest at low RPM with high power settings or in most cases, during acceleration). The higher cylinder pressures force the piston ring against the cylinder wall and allows the honing (the X shaped surface machined into the cylinder wall, similar to a very fine file) to seat the ring creating a very tight seal. This is done right after a new ring is installed (like on a new engine or top end work), once the edges of the honed surface is worn down it no longer provides this "filing" effect and then serves only to hold an oil film for lubrication.

Now for the non-ringed engine like what we have in our glow engines, ABC, ABN. since there is no ring to seat we dont have to worry about seating a ring, our only concern is the interference fit at the top of the cylinder. The cylinder is designed to be "zero" clearance at operating temperature, which allows an oil film to provide the seal we want. This is achieved during the first few min of engine operation. If you look at a zero time engine you will see the same "x" pattern honed into the cylinder that you find on a ringed engine and it serves the same purpose initially to "file" the piston, then to provide a surface to hold a lubricating film. The key to getting this proper fit is to start the engine, get it to operating temperature as quickly as possible, then allow the engine to run at stabilized temperature for as long as possible while the clearances are worn into the piston. Again once the edges are worn off the honed pattern they no longer provide this "filing" effect.

now, since I have time I can go through step-by-step how (and why) I break-in every engine I break-in.

With the engine at factory needle settings and full tank of fuel start the engine. Once its started bring the engine to full throttle and from now on thats where it stays until it runs its self empty. Once at full power lean the high end needle for peak RPM and maintain this setting for 10-15 sec. This brings the engine up to operating temperature as quickly as possible. We dont want to leave the engine this lean for more then 25-30 seconds, two things will happen. The engine will over heat, we wont be providing enough lubrication for the piston in the cylinder which will lead to scuffing, especially at this critical time in its life. After the 10-15 seconds richen (out) the high needle until the RPM drops roughly 1,000-1,500 RPM, you're looking for a rough running engine (the engine is actually starting to fire every other stroke. "four cycling"). Run the engine in this condition for about 1 min, this is to provide plenty of lubrication for the honing process. After about 1 min in this condition the engine is starting to cool off to below normal operating temperature and we need to get it warmed back up. Again lean to peak RPM and hold for 10-15 sec. Then repeat the rich running cycle for another minute. Repeat this process for the entire tank. At the end of the run and for a gallon or two afterword you will/may notice the oil on your airplane to be blackish. This is the aluminum from the piston as its being broken-in and it will go away. After this first tank you can fuel up the aircraft and restart the engine, adjust the high needle to peak RPM, then richen it back up until you note a drop of about 200 RPM and can see a trail of exhaust smoke. This is the perfect place to run the high end either for the rest of the engines life or if you're into high performance, until the blackish oil stops or 2 gallons, after-which lean to peak, richen for 50 RPM drop and let'er rip.

As far as idle/low needle. I've always set the high needle to where I'm gonna run and then let the engine idle for 30sec and then go to wide open as fast as possible. If the engine dies right away its too lean, if it sags or sputters its too rich. I'm looking for an engine that will go from an extended period of idle to full power with no hesitation or stalling. There are some that do the pinch test but i've found that setting still a little too rich for me. Once I get idle settings set, recheck high needle settings and if needed readjust (low will affect high but high wont necessarily affect low, depends on carb design) then recheck low... once both are good and no adjustments are necessary, go fly.