In the area of engine break-in, there are several different procedures that have been used over the years. Different manufactures, top drivers, and hobby store experts all vary in their opinion as to which is best. Over the past 10+ years of nitro racing I have found this procedure to give me good results in both areas of on-track performance as well as reliability.
Install the new engine into your chassis complete with clutch and ready to go as if you were going to race. Take a screwdriver, glow igniter, starter box, and a full bottle of fuel to a remote area where you won't annoy anyone with a running engine. Start your engine and allow it to warm up with several short ½ throttle “blips”. Once your engine has some temperature in it you can begin the break-in process. Start by opening the top end needle 1 full turn. Open the carburetor to full throttle and hold it there. Open the top end needle until the engine flames out. From here, close the top end needle ¼ turn and restart the engine again running it at full throttle. Run the engine full throttle for 3 tanks and shut the engine down.
Allow the engine to cool for 15-30 minutes. Once the engine has cooled down, run through the same process again for two additional tanks. On the third tank close the top end needle by ¼ of a turn. Instead of holding the engine at full throttle, start using the throttle by doing 2-3 second bursts of full throttle. Once you have done this for ½ of the tank close the top end needle another ¼ of a turn and continue throttling the engine. Continue closing the top end needle ¼ turn per every ½ tank of fuel. Do this until you have run a grand total of 8 tanks through your engine and then shut the engine down.
Allow the engine to cool for 15-30 minutes and then you are ready to hit the race track. Drive two tanks rich around the track slowly closing the top end needle sneaking up on that perfect mixture. At the end of the second tank your engine should be tuned to race speed. This will give you a total of 10 tanks of fuel through your engine before you really start to lean the mixture for ultimate power.
The reasoning behind this break-in procedure is that there is always a large volume of fuel flowing through the engine. Large volume of fuel means plenty of oil and lubrication for the break in process. This also helps flush out any burrs or small pieces of material that may have been left over from the manufacturing of the engine. Also, the engine has a load placed on it by trying to cycle all of the fuel through it – somewhat similar to the load that you will put on it out on the track when you try to power your car around the track. Last, the engine is broke in with rpm – if you break the engine in at idle then you will not be seating the piston, sleeve, and connecting rod at anywhere near the rpm that it will have to be ready for out on the race track.
This is the procedure that I have used with good success over the years of nitro racing. When I've hurried the break-in process I have had some engines last 30 minutes to 4 hours of racing. When I've taken my time and run through this process I've run engines as long as 16 hours of racing!! Yep, same piston and sleeve!!!! Be patient and have fun!!