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Nombre de messages : 299
Date d'inscription : 04/07/2011

MessageSujet: Conseils pour roder un moteur (en anglais...)   Ven 12 Aoû - 9:34

How to Break In your New Engine, by Ron Paris

Published on 06-27-2011 06:54 PM

It's all in the carb! Everything that’s needed to ensure your engine’s long life is contained in its carburetor. Most carbs have three adjustments that you’ll need to familiarize yourself with: The first, and most important, is the high-speed needle setting. This needle valve controls the mixture of fuel and air that enters the engine’s combustion chamber during high-rpm running. More fuel in this mixture causes a “rich” condition, while less fuel (more air) causes a “lean” condition.

A richer high-speed needle setting will actually help keep the engine running cooler as it reduces rpm, and because there is more fuel passing through the combustion chamber, a bit of “liquid cooling” actually takes place. A leaner setting will allow the engine to achieve higher rpm, but will also cause it to run hotter.

A properly set high-speed needle will provide a compromise between a cool temperature and high rpm. Needless to say extremes in either direction aren’t good, but nobody ever blew up their engine by running it too rich!

Many racing engines also have a secondary needle-valve adjustment that is used to control the engine’s low-speed fuel/air mixture. This needle is used to adjust the way the engine makes the transition from low to high rpm. It also helps control the fuel mixture when the engine is idling. A low-speed needle setting that is rich will reduce throttle response at low rpm, and may make the car sluggish coming out of tight turns. A leaner setting will allow the engine to make more power during low rpm operation.

Some sport-type nitro engines have eliminated the low-speed needle entirely. While this doesn’t allow for the highest level of tuning for the experienced racer, having one fewer needle to fuss with does make these engines much more user-friendly to the beginner, who only need set the high-speed needle.

Finally, there’s the idle-stop screw. Basically, this screw is used to keep the carb’s slide or barrel from becoming totally closed (which would stop the engine from running). A small opening of the carb allows just enough air and fuel to mix and keep the engine running. Setting the idle is usually done after you’ve properly set both the high- and low-speed needles.

Before you even place your nitro car on a starter box or yank its pull-starter, read the instruction manual that came with your engine or car kit. Most nitro kits’ manuals have a section that will tell you approximately where the carburetor’s needle settings should be for initial startup. We have found that higher-end (more expensive) engines tend to follow the instruction’s settings more closely, which means that when the manual says to set the high-end needle at three turns out from closed, this is almost exactly where the engine will run best for break-in.

For sport-type engines, the needle position that allows the engine to fire and run may be quite different from what’s recommended in the manual. In our experience, if a sport engine won’t fire, leaning the high-speed needle in small increments will get it going. Once the engine fires and runs consistently, we will usually richen the needle back to where the instructions recommend. Never run a new engine, or even an older one, too lean!

We know that you want to go out and do some hot laps the instant your new engine fires to life, but don’t! The first few runs of your new engine are critical. Once your engine starts, place the car on a stand so that its wheels can’t touch the ground. Let the engine idle at low rpm for a few minutes (two or three minutes will do), then shut the engine down and let it cool.

During this procedure, it’s also important that the piston not be at the top of the cylinder while the engine is cooling. Part of what’s taking place during break-in is that the engine’s mating parts are being heat-cycled. They’re expanding when the engine is hot and contracting when it cools. Heat cycling stabilizes the metal and allows mating parts to fit better against each other. Keeping the piston out of the upper portion of the cylinder (which is smaller in diameter than the lower portion in order to create a better seal during combustion) will allow the cylinder to properly contract as it cools—without interference from the piston. To figure out where the piston is in relation to the cylinder, just turn the flywheel. It will become difficult to turn when the piston is at the top of the cylinder, where the fit between the piston and cylinder is its tightest. Just turn the flywheel until the piston is in the middle of its “easy turning” part.

Now you’re ready to lay down some horsepower, right? Wrong! Repeat the above steps three or four more times.

After you’ve heat-cycled your engine, you can finally put it on the track. But don’t get too excited yet. You must run it with a very rich high-speed needle setting. Some engine experts recommend that, during break-in, the engine be set rich enough so that it will actually four-cycle instead of two (our nitro engines are two-cycle, which means that the fuel/air mixture is ignited once for every two strokes of the piston). Four cycling means that the engine is actually only firing one time for every four strokes of the piston. In this condition, all of the unburned fuel passing through the combustion chamber takes heat (and any tiny metal particles created during the breaking-in process) right out to the exhaust pipe!

Do you really need to run the engine this rich? Well, the experts know their stuff, but we have broken in dozens of new engines without actually allowing them to four stroke. Whether or not you four-stroke your engine during break-in is entirely up to you—just make absolutely certain that the high-speed needle is set very rich: lots of blue smoke should be coming from the exhaust, and the engine should sound “blubbery.”

During this procedure, it’s vital that you avoid prolonged use of full throttle, which could strain the engine. You should instead “blip” the throttle as you drive the car to avoid spending too much time in one particular rpm range.

Run the engine using these settings for three or four tank-fulls of fuel, allowing the engine to cool in between runs.

Once you’ve put about six to eight tanks of fuel through the engine (as outlined above), it’s time to begin leaning the high-speed needle and making some power! Begin by leaning the high-speed needle (by turning it inward, or clockwise) by about one-hour (if you imagine the needle as a clock-face, one full turn of the needle would equal 12 hours). Run the car for a minute or so, then bring it back in and lean the needle by another one hour increment. Repeat this process until the engine begins to achieve good rpm, but it shouldn’t be allowed to “scream” quite yet. The engine should still be creating lots of blue smoke from its exhaust.

Before you achieve that screaming race setting, we recommend that you run your engine for a few more tank-fulls in this “almost race” setting. Once you get the needle set to where your engine is making good rpm, richen it (by turning the needle counter-clockwise) by about a quarter of a turn. This is your final setting.

Once you’ve found a good setting for the high-speed needle that allows the engine to make good power yet still push plenty of blue smoke from the exhaust (especially when the car exits a turn), it’s time to set the low-end needle and the idle-stop screw.

Most engine manufacturers recommend a specific setting for the idle-stop screw, and they’re usually well within the ballpark. For now, set the idle-stop screw so that the engine will idle at a moderate rpm without stalling.

Bring the engine up to operating temperature by driving it for a few minutes. Now stop the car and listen to the engine’s idle speed. If the engine idles fast but then slows down in just a few seconds, the low-speed needle is probably set too rich. Lean the low-speed needle (by turning it clockwise in one-hour increments) until, after running a few more laps, the idle stays high for about twenty seconds or so when you stop the car. Once you’ve done this, use the idle-stop screw to make the final adjustment of the idle speed.

For sport engines which lack a low-speed needle, the idle-stop screw is the only method of adjusting the engine’s idle speed. For these engines, simply turn the idle-screw clockwise to increase idle speed, and counterclockwise to reduce the idle speed.

Setting your engine’s idle speed isn’t a contest to see how low you can get it without stalling the engine! Your goal when setting the idle should be to allow the engine to run at moderate rpm without the clutch being engaged whatsoever. Your car should be able to sit at a standstill when idling. If you have to hold the brake, the idle is too high. If you have to blip the throttle to prevent stalling the engine, the idle is too low.

Nitro powered R/C vehicles can be tons of fun, or they can cause tons of frustration. The difference between success or failure lies with the break-in process. If you follow these steps, have patience, and use your noggin, you’re assured of success. Rushing through the break-in procedure or worse, forgetting it altogether, is a recipe for disaster.

Tuning your engine is the hardest part of nitro racing. But once you’ve learned how the carburetor works, and which screw does what, it will all become second nature. So when you’re running your car, you’ll always know exactly what to adjust to gain the highest level of performance possible.

1. Rich is good. Blue smoke should always be coming from the exhaust.
2. Always set the high-speed needle first
3. Never try to tune a cold engine—wait for it to get up to operating temperature.
4. Always begin your engine tuning from a rich high-speed needle setting. Never start out with a lean setting.
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