Engine Break In
All new engines require some sort of break-in procedure. Engine break-in is of paramount importance in getting the proper life and performance out of your Duratrax R/C car engine. Without a proper breakin, your engine won't last very long nor be able to deliver its full range of performance, which include good full throttle performance, good acceleration, and a good idle. All of those necessary characteristics will not develop as they should until the engine's been broken-in.
To start the break-in process, you need to make sure the engine's carburetor is set to the starting points recommended by the engine's manufacturer. Check the engine's instructions for those settings, or the articles on engine setting listed on the Web site.
With both high-speed and low-speed needles at their initial starting points, which should be quite rich, start the engine. Leave the glow plug battery on the model. It should run, but have a bit of trouble coming up to speed when you pull the trigger to open the throttle. If the engine hesitates and seems to accelerate haltingly, then the mixture is quite a bit rich.
Start driving the car up and down the driveway or parking lot. Don't go off-road at this point. Also don't try to corner tightly. Accelerate the model up to about 3/4 throttle and let it coast a bit. Repeat this several times, and then bring the car back to you and lean the high-speed mixture a bit. About 1/8 turn should do it. Do a few more runs like this.
Now keep the needle at the same point, but accelerate up to full throttle. After five or six passes, you can lean the high-speed needle a bit more, until the engine just stops hesitating when you open the throttle. A slight stumble is acceptable.
You can now bring the car to you and start to adjust the idle mixture. Use the "pinch test" to determine whether you should lean or richen the mixture. Give the fuel line a pinch of about one-half second or so and listen to the engine. If it speeds up, you can lean the idle mixture a bit. If the engine slows down or stays the same, richen the idle mixture a bit. Don't turn it more than 1/8 turn at a time. You can tell the idle mixture is just about right if the engine speeds up very slightly when you give the fuel line that short pinch. You can also tell that the idle's adjusted correctly when you get a clean acceleration when you open the throttle.
After 3-4 tanks' worth of running, you should be able to lean your engine to a point just rich of its peak RPM setting. You can tell that the engine's on the rich side of peak RPM when you can accelerate to full throttle and the engine will then hold RPM. If it runs up to a point and then sags down, the engine's too lean. Immediately slow down, bring the car back to you, and richen the high-speed needle at least 1/4-turn. You'll then need to readjust the high-speed needle valve as detailed above.
Too-lean running will quickly damage your engine because a lean run means that the amount of fuel going through the engine is on the low side, and since the lubricating oil is in the fuel, lubrication is on the low side. Add to this fact that many R/C car fuels have low oil content and you can see that a lean run is not a good thing.
Too-lean running can cause the connecting rod to seize to the crankshaft. This will cause it to break and scatter metal bits throughout the engine. Too-lean running can also cause an overheat. Overheating can cause the piston/cylinder fit to become too loose because when an overheated piston cools off, it will shrink to a slightly smaller size. The loose fit will cause compression loss.
You can tell you have low compression because the engine will not start easily and either idle poorly or not idle at all. You may not even get it to start if the piston/cylinder fit has been damaged badly enough. The only solution to this problem is to install a new piston/cylinder assembly and start the break-in process all over again.
When the engine's properly broken-in, you should have the engine setup so that only a small amount of adjustment of the high-speed needle is needed each time you go out to run. Just turn the high-speed needle about 1/4-turn counter clockwise before the first start of the day, allow the engine to warm up, and then drive it a bit to get the high-speed needle set. Usually, you won't have to set the main needle for the rest of the day, once you have it set during the first run.
When racing, the practice laps are used to get the engine's mixture adjusted correctly. It should be OK for the race that follows.