ehroof -> Nitro Engine Troubleshooting and Tuning (12/29/2006 2:47 AM)
Since i have seen alot of engine trouble in the last couple of days i thought this may help out. I have included a picture of the tuning needles and idle screw at the bottom.
It's a fact of nitro life: sooner or later, your engine won't want to start or stay running long enough for you to get up on the drivers' stand. Don't shelve your piston-pounder and start charging batteries just yet; hard-starting woes aren't usually tough to fix. Follow these steps, and you'll reach nitro nirvana again in no time.
Before we start troubleshooting, let's first make sure the piston and sleeve aren't toast. Remove the engine's glow plug, and rotate the flywheel with your finger. As the piston reaches top dead center, you should at least feel resistance as the piston is “pinched” by the sleeve. This tight fit seals the combustion chamber and is critical to engine performance. If you don't feel any pinch, you probably need to replace your piston and sleeve.
Check out the glow system
Even rank novices know to check out the glow plug when starting problems occur, but few remember to check the other half of the glow system—glow starter! Make sure that your glow starter is fully charged (or has a fresh battery), and its contacts are clean. Press the glow plug into the glow starter and watch the coil; it should almost instantly glow bright orange, and the entire coil should glow. If the coil glows dull red, or it doesn't glow at all (and you're certain the glow starter is charged and making good contact), you need a new plug.
If you have an electric starting system, the testing procedure is the same, but you must remember to touch the glow plug to the heat-sink head as you crank the engine; if you don't, the glow plug won't light because the circuit is incomplete.
Do the tighten-up
Make sure that the engine's heat-sink head and backplate screws are fully tightened. Cinch them down in the pattern shown to ensure even tightness; if you torque the screws down completely one at a time, you'll warp the parts and prevent them from sealing properly against the engine. Pull-start engines require the starter housing to be removed for backplate access; be careful not to uncoil the starter spring in the process! Slip a screwdriver between the backplate and housing to prevent the spring from popping out, then tape or rubber-band the assembly after removal so it doesn't go “boing!” on your bench.
Speaking of sealing...
While you’re wrenching, take a look at the gaskets or O-rings that seal the carburetor base and backplate. If they’re damaged, replace them. A coat of Permatex Ultra Copper sealant (or similar sensor-safe automotive sealant) is also good insurance against air leaks.
Know your limits
If your engine seems to start easily enough but shuts down as soon as you let off the gas, check your idle-limit screw—the little guy just in front of the carburetor opening. Turn the screw so the carb still has a 1mm (or so) opening when full brake is applied. If the screw turns very easily, put a drop of thread-lock on it to prevent engine vibration from causing the setting to drift.
If you went nuts with your carb’s needle settings before your starting troubles began, you should reset them to factory specs. Lost your manual? For most engines, a good starting setup is “flush” for the low-end needle (adjust the needle so its screw head is level with the opening in the carb body the screw head sits in), and “two turns out” for the high-end needle (gently turn the needle clockwise until it bottoms out, then give it two full counterclockwise turns).
How’s that fuel tubing?
Even tiny pinhole leaks in your vehicle’s fuel and pressure lines can cause erratic engine operation and starting difficulties. When in doubt, replace the fuel tubing; it’s inexpensive, and with all the color options available, it’s a cheap and easy way to give the chassis a new look.
Tanks for the memories
Your fuel tank isn't clogged, is it? Look for any crud in the tank where the fuel pickup line is, and get it out of there. Likewise, you should check your vehicle's in-line fuel filter (if so equipped) frequently; it can become clogged with debris and impede fuel flow if not cleaned regularly. Finally, consider removing the tank's plunger-type primer, if so equipped; such primers are prone to air leaks. After you've removed the primer, fill the hole with a machine-thread screw and some silicone sealant.
Freshen up your fuel
If your jug of fuel spent the winter on the garage floor, it has probably gone bad. Very bad. Dispose of it responsibly, and get yourself a new gallon. Store it in a cool, dry place away from sunlight and off the floor. If you like to race outside or go on extended-play missions outdoors, try to keep your fuel out of the sun and heat. Trinity's Nitro Kooler bags are the best way to prevent solar heating and protect fuel in clear containers from light exposure.
Now that you've ironed out any potential problems, you're ready for a first-pull startup. Here's how to make it happen:
1. Prime the carburetor. Remove the pressure line from the exhaust pipe, and blow into it; you'll see a solid jet of fuel fill the fuel line up to the carburetor. When the fuel hits the carb, stop blowing. Reinstall the pressure line.
2. Install the glow igniter. If it's a cam-lock type, make sure it's secure. Is there a gauge on top? Make sure the needle is in the green.
3. Crack the throttle. Instead of triggering the radio, just turn the throttle-trim knob to open the carb another 1/2 millimeter or so.
4. Pull the starter cord! Give a sharp tug, and your engine should now be running. At the very least, it should pop on the first pull and start on the second or third (the extra tugs are usually only required if the carb wasn't fully primed).
MAXIMIZE YOUR ENGINE'S POWER POTENTIAL
So now you can get your engine started on a regular basis, but you’re still struggling with the fine-tuning that will score you a win at the racetrack (or bragging rights at the parking lot). Properly tuning a nitro engine can make that difference without jeopardizing its health. It takes time to learn how to really tune your engine, however. There’s a certain “feel” to how your car drives and a certain sound you’ll come to know when your engine has been ideally tuned. Other cues that you feel and hear tell you what to adjust when your engine isn’t running properly. When it’s time to tune your engine, there’s no substitute for plain old experience. Reading about engine tuning is helpful, but you need to experiment with your engine to improve your tuning skills. The good news is that the following tips will help you avoid some of the pitfalls of fine-tuning and achieve tuning proficiency more quickly.
BASE-LINE MIXTURE SETTINGS
Engine manufacturers often include base-line settings for the mixture needles, so it’s wise to start with these. If this information is not provided, then you must arrive at needle settings that will get the engine started. A universal starting point is usually about 1 turn open (counterclockwise) on the low-speed needle and somewhere in the 2- to 3-turn range on the high-speed needle. This varies among engines, but it gets you started running, and then you can make the necessary corrections. After the initial startup, follow the proper break-in procedure, then worry about performance tuning!
The proper sequence for adjusting the mixture needles is hotly debated. When you start to fine-tune the engine, it’s generally best to start with the high-speed needle, then set the low-speed. First, however, get your engine running, and keep it running before you worry about race tuning.
During break-in, the engine typically idles a long time, so it’s best to adjust the low-speed setting first so the engine runs slightly rich (loading up every 30 seconds or so). It requires an occasional “blip” of the throttle to clear out any raw fuel that has accumulated in the engine. Once break-in is finished, then get the high-speed needle in the ballpark.
Place the car on the ground and accelerate smoothly to give the engine a chance to build some heat. With the high-speed needle in the proper range, the engine should be able to rev relatively well up to full speed once it has been running for a few minutes on the track or parking lot.
A word of caution first: there’s a fine line between the perfect tune and a blown or damaged engine. Nitro-engine fuel also contains engine lubricant, so as you get close to dialing in the mixture to where there is just enough fuel to burn and deliver maximum power, you also are close to having just enough oil to keep the engine lubricated. Anyone with experience in tuning 2-stroke engines can tell you that they run best right before they seize or blow up. Our engines are a little tougher and more capable of taking some abuse than bigger 2-strokes, but there’s no sense in pushing the mixture settings so lean that you risk damage to the engine. I can’t say this too often: get the engine up to full running temperature by running the car exactly as you would on the track or parking lot. The high-speed needle setting depends on the type of driving you do. I’ll start with a racing setup.
Racers will tune the high-speed setting to get from point A to point B as quickly as possible. (Performance also depends on a proper low-speed needle setting, but for now, let’s concentrate on the high-speed setting.) The best place to race tune your engine is on the track where you run. I prefer to set the high-speed mixture so the car can leg out the track’s longest straight section in the shortest time. Estimate the time by “feel” and gut instinct, or use a stopwatch for more accuracy. The high-speed mixture should be set to maximize engine performance for that particular track configuration. A short, tight track may require a main mixture setting just a shade on the lean side to provide maximum power out of the corners. You needn’t be concerned about high-rpm performance because the track is too small for the engine to ever reach peak rpm. A long, high-speed track may require a slightly rich main-needle setting. If an engine constantly revs at the upper limits of the rpm range, the fuel mixture should be richened to ensure proper lubrication across the entire rpm range. This slightly rich setting might reduce bottom-end acceleration to a degree, but longer tracks require a slightly richer mixture setting to let the engine rev to its limits without running dry of fuel and oil.
Why is it important to tune the engine to the track? A mixture needle can only provide optimum performance within a relatively narrow rpm range. Anywhere below this hypothetical rpm range, the engine runs slightly rich; anywhere above, it gets progressively leaner. Until we have fuel injection that constantly optimizes fuel mixture throughout the rpm range, there needs to be a degree of compromise with the mixture settings. So, ideally, set the mixture to provide the most power in an rpm range that is best suited to the track on which you run.
Running in parking lots, particularly larger ones with a lot of breathing room, requires unique mixture settings. It’s a common mistake to establish mixture settings for maximum punch—as though the car will be run within the confines of a tight racetrack—and then to go out and run at wide-open throttle (WOT) in a huge parking lot for 5 minutes. This type of running is incredibly unhealthy for an engine to begin with, and compounding the problem with an excessively lean main-needle setting is a recipe for disaster. If you still insist on torturing your engine, the high-speed mixture setting needs to be as much as 1/4 turn richer than typical to provide optimum fuel for the upper rpm range. The engine will be a little softer when accelerating from a standstill, but it’s the only way to ensure there is an adequate supply of fuel and oil when running at the upper end of the rpm range. The inherent danger is that this type of running taxes the connecting rod and other engine components to their limits, but having the proper mixture setting will at least delay the inevitable.
The high-speed needle is dialed in, so now let’s properly set the low-speed needle. It’s important to set it last because it simply regulates the fuel that flows from the main needle at low throttle settings. Lean out the main needle, and you automatically lean the low-speed needle as well. For this reason, it’s wise to finish with the low-speed setting.
Again, it’s imperative for the engine to be at full operating temperature. There are many methods of testing the low-speed needle setting; one is to pinch the fuel line. When you pinch the fuel line, the engine rpm increase slightly. Keep pinching it, and the engine will eventually stall. If the engine rpm increase dramatically, it indicates that the low-speed setting may be too rich. Or, if the low-speed setting is already too lean, the engine rpm may not increase much at all, and the engine will stall rather quickly. It’s a somewhat crude method and doesn’t tell you what to expect from the engine on the track, but it will get you into the ballpark.
Another common way is the “see-how-long-it-will-idle” method. The low-speed needle adjustment affects how long the engine will idle. A too lean fuel mixture causes the engine to race and possibly stall, limiting the duration of a steady idle. A too rich low-speed-mixture setting causes the engine idle to steadily drop and eventually stall. The ideal setting allows the engine to hold a smooth, steady idle for 10 to 20 seconds (max), and then the engine rpm decrease steadily because the crankcase loads up with fuel. Why? There are no awards given for the longest-idling engine. If the engine is able to idle steadily for a longer time, then it may start to lean out and heat up during a race and make it difficult to drive the car and keep the engine running. The only flaw in this method is that it doesn’t tell you whether you have an artificially rich mixture to compensate for an idle speed that’s too high.
A common mistake is to set the idle-speed screw to keep the carburetor open too far. The low-speed needle must then be artificially rich to bring the idle down to a reasonable rpm. The symptoms are similar to a too rich low-speed-mixture setting; there’s just a delay in the loss of engine rpm. How do you avoid this? This is also something that becomes easier with experience, but just continue to reduce the idle speed and lean the mixture until you know you can’t go any further. Bottom line: adjust the idle-speed screw to suit the fuel-mixture setting, not the other way around.
The simplest and most foolproof method to properly set the low-speed mixture is, again, to do it on the track. Set the low-speed needle so your car gets the strongest launch after sitting still for about 10 seconds. The engine should be able to pull strongly off the line without hesitation. A noticeable hesitation might be the result of either a rich or a lean low-speed mixture; knowing the difference takes experience, but look for signs that help point you in the right direction. How an engine decelerates can tell you as much as how it accelerates. If the engine spools down and rpm drops uncharacteristically low, it indicates that the low-speed-mixture setting is too rich. Or, if the engine takes too long to reach a steady idle and seems to want to keep revving, that tells you the low-speed-mixture setting is too lean. It can also indicate a lean high-speed-mixture setting, but that setting should have been addressed by properly setting the high-speed mixture first.
It will take a little time to get it right. If you make small adjustments and are patient, you really can’t do anything wrong. An adjustment you make in the wrong direction is reflected in engine performance; to correct the problem, simply go the other way.