By the way Encan, these little Cox engines don't really need any break in to speak of. A slightly rich off peak first run and then go for broke. It's good if you avoid running lean for the first 1/2 dozen or so runs though.

If you broke it in using the classic big engine method of sloppy rich and slowly leaned it out over a number of runs then it may be varnished up as fastlash suggests. If the insides of the cylinder are a brown color instead of just grey steel then that is the case. Clean it out with a little fine steel wool and lacquer thinner or acetone. Use gloves, that stuff is nasty on your skin or use the steel wool wrapped around a little bit of stick to avoid the solvent. DO NOT USE ANYTHING LIKE SANDPAPER OR OTHER ABRASIVES.

Oh, you can also use a fine abrasive like toothpaste or a fine household cleaner like VIM. If it feels gritty to your fingers then it's too coarse and you should not use it. If you use these products then use them lightly and check often. Stop as soon as the brown is gone. A stick of balsa is a good scrubbing tool for the toothpaste. Remove the glow plug and then the cylinder from the rest of the engine using the tools you got with it. Do not use anything that squeezes the cylinder. The steel is soft and WILL bend ruining your engine.