How to Tune a Nitro RC Car


Today's RC car nitro engines are more reliable and easier to tune, but many newcomers still struggle when it comes to tuning a carburetor properly. Tuning RC car engines is actually easy once you grasp the concept that nitro engines require a balance of air and fuel to run properly, and that temperature and humidity effect how the engine performs. Your engine must be properly broken-in before you attempt to follow these steps.

Things You'll Need

  • 1/8-inch flat blade screwdriver
  • Nitro fuel
  • Extra glow plugs
  • Temperature gauge (optional)
  • Medicine dropper

Carburetor Adjustments

  • Most nitro engines have three tuning adjustments located around the carburetor: the high-speed needle-valve, low-speed needle-valve and the idle-speed screw. The high-speed needle-valve adjusts the air/fuel mixture when the engine reaches maximum RPM. The low-speed-needle-valve adjusts the air/fuel mixture when the engine is idling and during acceleration. The idle-speed screw adjusts the idle speed. The high-and-low needle-valves and the idle-speed screw must all be adjusted properly for the engine to run reliably and reach maximum performance.

  • You'll need a flat blade screw driver--preferably one with a 1/8-inch wide blade and a long shank--to tune the carburetor. Turning the needle-valve clockwise leans out the air/fuel mixture and turning counterclockwise richens the mixture. Turning the idle-speed screw clockwise raises the idle and turning counterclockwise lowers the idle.

  • Always make small adjustments. Test the car after each minor adjustment to witness the effects on performance. Be patient and you'll be rewarded with a fast and reliable engine.

Tuning for Performance

  • Always adjust the high-speed needle-valve first. Start the engine and drive the car around for about a minute to warm up the engine. Next run the car down your street at full throttle. Bring the car back and lean the high-speed needle-valve by turning the adjustment screw clockwise 1/8 of a turn. Take another high speed pass down the street. You'll probably notice that the car is faster on the top end. Continue leaning the high-speed needle-valve 1/8 of a turn at a time and testing the adjustment by running the car down the street at full throttle. The car should become faster and faster until it reaches a point where it starts to hesitate or sound like it is suffering from fuel starvation when it reaches maximum RPM. This is a sign that you have leaned out the air/fuel mixture too much, so bring the car back and richen the mixture by turning the high-speed needle valve counterclockwise 1/8 of a turn. At this point, your high-speed needle-valve should be tuned correctly.

  • Adjust the low-speed needle-valve. With the engine running, apply the brakes and allow the car to idle for around 20 to 30 seconds. This will load up the carburetor with fuel. If the engine stalls before the 30 seconds are up, the needle-valve is set too rich. If it doesn't stall, stab the throttle and keep an eye on how the car accelerates. If the engine bogs down for a moment before taking off, the low-speed needle valve is set too rich and will need to be leaned out. Close the needle-valve by turning the adjustment screw clockwise 1/8 of a turn. Repeat this process a few times as necessary to get the low-speed needle-valve set properly. You'll know when the needle-valve is set properly because the car will accelerate quickly without any signs of bogging down. If it hesitates or sounds like the engine is deprived of fuel, you've probably leaned the needle-valve too much. You'll need to richen the mixture slightly by turning the adjustment screw counterclockwise.

  • As you adjust the needle-valves, you'll notice that the idle speed will increase gradually each time you lean out the air/fuel mixture. You'll need to turn the idle-speed screw counterclockwise to lower the idle speed each time you make an adjustment. Ideally, you want the engine to idle just enough to keep it running without engaging the clutch and forcing the car to move forward.

Tips & Warnings

  • Keep an eye on engine temperature at all times. If you're serious about the hobby, you'll want to purchase a temperature gauge. Quality temperature gauges are available at most hobby shops for under $40. When the engine is set properly, it should run at a temperature between 190 and 230 degrees F. Running an engine for long periods at a time at temperatures above 230 degrees will prematurely ruin it. If you cannot afford a temperature gauge you can test engine temperature with water. Use a water dropper, like the kind that is used to give babies doses of medicine. To test the temperature, make a few high-speed passes to bring the engine up to running temperature. Apply a couple drops of water on the cylinder head and watch how long it takes for the water to evaporate. If the water bubbles up immediately and evaporates in under two seconds, the engine is running too hot. If the water sits on the cylinder head and takes more than six seconds to evaporate it's running cold. You'll know when the engine is running at the proper temperature when the water evaporates in about three to five seconds.
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