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Tune Your Engine
I can't tell you how many times I have seen people run improperly tuned nitro cars at the track. If you aren't careful, you could blow that expensive engine up! If it's running too lean, consider the piston and sleeve blown, and if you run it too rich, you won't even be in the race. The same holds true for you backyard bashers out there. Here are three easy-to-follow steps to tuning your engine. They work for me, and they will work for you.
STEP 1 TUNE THE HIGH-SPEED NEEDLE
This is first thing to set when you tune your engine. The high-speed needle regulates how much fuel enters the carb, and it affects how you will set the low-speed needle. Basically, you tune your engine for temperature, so start the engine, and run your vehicle to get the engine up to operating temperature. You can use the "spit" test or a temp gun to check engine temp. In a "spit" test, the water you place on the head of the engine should boil off within 2 to 3 seconds. If it happens any faster, your air/fuel mixture is too lean; if the water doesn't evaporate, it's too rich.
When you use a temp gun, you want the engine's temperature to read around 220 to 230 degrees F. Some engines run better a little hotter. If your engine temp is more than 260 degrees, your mixture is too lean, and if it's less than 190, it may be too rich. Engines differ, and temperatures vary, but try to maintain temps in the aforementioned range, and pay close attention to performance; it's the best indicator of a good mixture setting. When you adjust your high-speed needle, turn the needle an 1/8-turn at a time. To richen the mixture (let more fuel in), turn the needle counterclockwise; turn it clockwise to lean it (let less fuel in).
When you use a temp gun, you want the engine's temperature to read around 220 to 230 degrees F. Some engines run better a little hotter. If your engine temp is more than 260 degrees, your mixture is too lean, and if it's less than 190, it may be too rich. Engines differ, and temperatures vary, but try to maintain temps in the aforementioned range, and pay close attention to performance; it's the best indicator of a good mixture setting. When you adjust your high-speed needle, turn the needle an 1/8-turn at a time. To richen the mixture (let more fuel in), turn the needle counterclockwise; turn it clockwise to lean it (let less fuel in).
STEP 2 TUNE THE LOW-SPEED NEEDLE
Now that your high-speed needle is set, you can attack the low-speed needle. This is sometimes easier to do with the body off the vehicle. First, drive your vehicle around for a couple of seconds, and then bring it in. As soon as you stop the vehicle, pinch the fuel line. If your low-speed needle is set properly, the engine rpm should slowly increase after a couple of seconds and then slow down till the engine stalls (if you're careful, you can let go of the fuel line before the engine stops). If the engine dies very quickly, the low-speed mixture is too lean, and if it takes more than 5 seconds for the engine to stall, it's set too rich. Don't let the engine sit too long before you pinch the fuel line. The engine will load up with fuel, and it will seem to be running rich. This needle doesn't require as many tweaks as the high-speed needle, and only the slightest adjustments have to be made. If the setting is way off, you may also have to adjust the idle screw to keep the engine running at idle. A lean low-speed setting will make the engine idle high, and a rich one will cause the engine to idle low.
What the heck do these needles do?
Ok, you hear all this talj about high and low-speed carburetor needles, but you aren`t sure what each needle does. Here are the basics you see the screw where the fuel enters the carb? That’s your high speed needle. The low-speed needle and idle screw may be a little harder to find, depending on how your engine`s carb is set up. And if your engine has a third needle, it can be more confusing. Basically, you never have to touch the "third" needle. This is just a replaceable nozzle, and the factory setting on this part is usually fine. The low-speed needle is usually in the centre of the arm that works the carb. Some 2-needle slide carb's have the low-speed needle on the opposite side. The idle screw on a rotary carb is on the front of the carb with the head pointing up. The idle screw on a slide carb is also found on the front of the carb, but the head is off to the side.
Here's the high-speed needle; it affects the operating temperature of your engine. A good temp range is between 220 and 230 degrees F.
The low-speed needle doesn't have to be adjusted as often as the high-speed needle. Sometimes, it might not need to be touched at all. When you set the low-speed needle, pinch the fuel line and listen to the engine; the rpm should slowly rise and then fall.
Here you can get a good look at all the needles on this carb. The high-speed needle is on the top right, the low-speed needle is in the middle of the arm and the idle screw is on the front.
Low-Speed
High-Speed
