Nitro engines combust fuel internally to generate the energy necessary to move a radio-controlled (RC) vehicle. While this action is generally regulated by using the hand-held transmitter to control the throttle, there are ways of adjusting the physical components of the engine itself. Learning how to adjust your HPI racing carburetor will allow you to fine-tune your nitro engine for optimal performance.
Turn the high-speed needle until it is completely closed, then rotate it 2-1/2 turns counterclockwise. The high-speed needle appears on the left side of the carburetor, and it resembles a stack of two ridged cylinders.
Move the throttle lever to 15-degrees open, which equals around 20 percent of throw. These terms refer to the position of the the lever, and how open or closed the setting is on the carburetor. A 15-degree-open setting will supply the extra fuel necessary for starting the engine.
Turn the mixture control screw, adjacent to the throttle lever, 3-1/2 turns counterclockwise from closed. This is the factory break-in setting.
Rotate the throttle stop screw (adjacent to the high-speed needle) 1-1/2 turns clockwise from closed. Measure the opening to check that it is about 1.5 mm wide. Use a ruler with millimeter increments, hold it out in front of the stop screw and check that either side of the opening aligns with the millimeter side of the ruler. Count the number of millimeters that cross over the gap; it shouldn't be more than two.
Rotate the high-speed needle 2 to 4 clicks clockwise until the smoke from the exhaust pipe is light during high speed. Turning the high-speed needle clockwise leans the mixture of nitro and air by allowing proportionately more air to fuel. If there is no exhaust smoke at high speed, rotate the high-speed needle 2 to 4 clicks counterclockwise to richen -- or add more fuel -- to the mixture.
Drive the vehicle at top speed, then let off the throttle to allow it time to return to idle. If the vehicle doesn't return to idle, reduce the throttle servo's settings by using the transmitter's throttle trim. Turn the throttle trim counterclockwise to reduce it, and clockwise to increase it.
Set the mixture control screw by assessing your vehicle's behavior. Allow it to idle for 5 seconds, then apply full throttle. Rotate the screw 1/8 turn clockwise if the engine hesitates or expels dense smoke from the exhaust. Rotate the screw 1/8 turn counterclockwise if there is no smoke coming from the exhaust -- or the engine accelerates and then cuts off.
Adjust the throttle stop screw to compensate for an engine that idles at a fast pace even when the throttle lever is closed. Rotate the throttle stop screw 1/8 turn counterclockwise to reduce the gap.
When closing the high-speed needle, use gentle force; tightening it too much could permanently damage the valve.