When tuning an RC nitro engine on any RC vehicle, it is important to understand how adjusting the tuning screws will impact engine operation and performance. The goal of tuning the engine is to maximize the speed and acceleration performance without damaging the engine.
The default engine carburetor settings from the manufacturer are set for a very rich fuel- air mixture which is required for engine break-in. Once the engine has completed the break-in process you will need to tune the carburetor settings to maximize the performance of the engine.You may also need to do some minor engine tuning when there are significant outside temperature changes, humidity, or altitude/ barometric pressure changes, or if you have changed the air filter, exhaust manifold components, fuel type, or glow- plug type.
In this blog, we are going to provide a brief overview of fine-tuning methods for nitro gas engines. We will explore the functions of high-speed and low-speed needles on the carburetor, what happens when you lean the fuel = air mixture, and what happens when you make the fuel-air mixture more rich.
Many people struggle with tuning the engine because they do not understand the concept of running too lean or too rich. To lean out or richen, an RC engine refers to the mixture of fuel and air going into the engine. Lean means to add more air to the air= fuel mixture. Rich means to add more fuel to the air -fuel mixture. The right air and fuel mixture will keep your RC's engine running properly and increase performance.
Base-Line Mixture Setting
It is highly recommended you keep a record of the base-line or default settings for the mixture needles as they came from the manufacturer. This allows you to return to these settings in case your adjustments get out of control. The way to record this, is to carefully count how many turns it takes to screw the high speed (and low speed) mixture screw to the bottom, record the # of turns (i.e. 3.5 turns) and then turn the screws back counterclockwise by the same number of turns.
When you begin to fine-tune your engine, you should start with the high-speed needle and once the setting is completed, then adjust the low-speed needle. Ensure you engine is warmed up before you start tuning.
Adjusting the High-Speed Needle
Both the high and low speed are precision tapered screws that allow you to control the fuel-air mixture.The high-speed needle screw controls the fuel-air mixture for the mid to high RPM zone of the engine. When you are turning the screw clockwise, you are giving the carburetor less fuel (i.e. making the fuel-air mixture more-lean). Similarly, when you turn the screw counter-clockwise, you are giving the carburetor more fuel (i.e. making the fuel-air mixture richer)
After engine break-in, you will need to make the fuel-air mixture more-lean by turning the high-speed needle screw clockwise (½ turn initially) and then do a test run to observe the performance and make subsequent adjustments of about 1/8 turn and again observe performance. If you do not see any further improvement in performance – do not adjust it further. Ensure you are still seeing smoke from the engine exhaust when it is running at high speed – if not, it is running too lean, and you will need to turn back the setting by 1/8 of a turn or more until you easily see visible smoke.Do not operate the engine if it is running too lean as you may damage the engine and the engine will be prone to cut out or stall. If the outside temperature is significantly lower than the last time you tuned it, you should check that it is not running too lean.
Adjusting the Low-Speed Needle
The low-speed needle controls the fuel-air mixture for the idle to mid RPM range for the engine, and therefore will impact how quickly your vehicle accelerates. This mixture is also impacted by the high-speed needle settings, which is why the high-speed setting should be done first
The low-end mixture needle (in throttle lever) typically requires only minor adjustment. The best way to check the low speed adjustment is by listening to the way the engine revs from idle. If the engine sputters, the low speed mixture is too rich. If the engine idle rises when you turn in the low-speed mixture screw a bit, the bottom end is lean. If it cuts out, it may be running too lean. When making any needle adjustments make sure you do it in small adjustments (1/8 turn).
Another way of adjusting and testing the low-speed needle setting – is to pinch the fuel line and inch or two before the carburetor. When you pinch the fuel line, the engine's RPM should rise slightly and run for 3-5 seconds before dying. If it runs longer it is running to rich and we will need to turn the low-speed needle screw clockwise ( 1/8 turn or less) , If the engine runs for less than 3 seconds it is running too lean , and you will need to turn the low-speed needle counterclockwise ( 1/8 turn or less)Do a quick test run between adjustments and keep tweaking it, until the engine stop time is in 3-5 seconds.
Adjustment at Idle Speed
There is a screw located underneath the air filter on the carburetor. It is either on the front or the back of the carburetor (dependent on the carburetor position of your nitro RC). The screw controls the idle speed of the engine when the carburetor is closed.
The idle speed should be just fast enough to keep the engine from stalling, and not too fast that the vehicle wants to move when it is sitting on the ground.If you remove the air filter, you should see an opening of about1.5- 2mm even when the carburetor is in the closed position
Turn the idle screw counter-clockwise to reduce the idle speed. If your engine stalls or cuts out, turn it clockwise to increase idle. Remember, the goal is to reach the point that is just before the engine stalls and cuts out.
By adjusting the fuel/air mixture, you can adjust the optimal temperature of your RC engine. Generally, the temperature between 98° to122° Celsius (210° to 250° Fahrenheit) around the glow plug area is best for most nitro engines.
Hot weather requires a leaner mixture setting – cold weather requires a richer setting. Most people make the mistake of treating the mixture needle like a thermostat, which makes them do the opposite – it is wrong to assume that the heat needs to be trapped into the engine in the cold weather, so a leaner setting is required and vice versa.
Cold air packs more oxygen into the engine, so going from hot weather to cold needs an increase of fuel to balance ratio.
Altitude can be a significant factor when it comes to tuning your nitro RC. The general formula for power loss with an increase in the altitude is 2-3% for every 1,000 feet above the sea level.
Air is thinner at higher altitudes; it leads to less fuel-burning oxygen than at the sea level. Notice that less air (oxygen) means less fuel to maintain the proper ration of air/fuel. So, running at higher altitudes require a leaner mixture setting.
Good additional References for tuning: