Needle valves are like a garden hose spigot. Open the spigot for more water or close it for less. The same holds true for the fuel, the more you open the needle, the more fuel will flow into the carb.
Imagine that the head of the needle valve—the part that looks like a screw head— is a clock face. That screw head gives you twelve hours per one full turn. When tuning, turn the needles in one-hour increments and allow 30 seconds for the adjustments to take full effect. Always tune from rich to lean. If you’re unsure where to begin and you’ve already altered the needle, richen the main needle two full turns from its fully closed position.
The idle screw is usually overlooked when talking about tuning. The idle screw sets the gap in the carb venturi and regulates the air that enters the carb with the throttle closed. The gap is just as important as the fuel you put in your tank. The gap should be set at about 1mm, or roughly between .9mm and 1.1mm. If the gap is wider, it will trick you into running the low speed needle (LSN) setting too rich to drop the idle. Not good, because the engine will constantly load up with fuel and stall. If the gap is too small, it will cause you to run a lean LSN to get the idle up. Not good, this will cause your engine to run hot and fall out of tune. At 1mm your engine should start and idle.
Start your engine and give it some throttle to clear it out. Now apply the brakes and listen to it idle. If the idle is high and the car wants to move then the LSN is lean. Richen the LSN one hour or until the idle drops to normal. It should idle low enough so the clutch doesn’t engage. If the engine idles low and stalls then the LSN is rich. Lean the LSN one hour and run the engine to allow the setting to take full effect. Apply the brakes and listen to the idle. You want it to idle for three to five seconds before the idle drops and starts to load up. This will give you a slightly rich LSN setting.
After you tune the high speed needle and get the engine up to temperature you can fine-tune the LSN and idle screw for more bottom end and throttle response. Running a slightly rich bottom offers longer engine life and lower engine temperatures. Extending engine life is always good! Just remember the idle screw and LSN work with each other and if one is adjusted the other will require some adjustment to
keep them in sync. Once your idle screw and LSN are set you won’t have to adjust them as often as the high speed needle.
The High Speed Needle (HSN) is the main needle on the carb, or by some instruction manuals is just known as the “main needle.” Here we’ll call it the HSN; this needle is used to adjust the amount of fuel at high rpm. Start by warming the engine up for two minutes by doing some laps on the track or just blasting through the yard. After it’s warmed up listen to how the engine sounds on the top end. It should have a slight high pitch sound at high rpm and should have a thin trail of smoke out of the tuned pipe. If it doesn’t seem to clear out on the top end then lean the HSN. Adjust in one-hour increments and always run the engine for 30 seconds to allow for the adjustments to take full effect. Lean the engine until it accelerates smoothly and performs the way you want with a nice stream of smoke from the exhaust. It should smoke at idle and high rpm.
Now after you have tuned your engine; check the temperature. Do this by using a temp gun from Pro Exotics or OFNA, or by using an onboard temp gauge like the Venom. A temperature gun or gauge is a great investment for your nitro engine and will last you for many years. Knowing the true temp is always better then guessing and is far better than relying on the water drop test noted below. Try to keep the temperature between 220-270 degrees Fahrenheit. Most engines will operate around 230-250 but you will tend to see higher engine temperatures in hotter weather and cooler ones in cooler weather.
If you don’t have a temp gun or gauge put a drop of water on the head after the engine is warmed up. It should take 3-5 seconds to boil off. If it jumps off as soon as it hits, it means you’re running hot (i.e., lean). Richen the HSN two hours and allow the adjustment to take effect. If the water sits there it means you’re running cool and you could lean the HSN. Only do this if you want more power. Remember keeping a slightly rich setting will always offer longer engine life and lower engine temperatures.