Throttle Valve Adjustment


There are several engines on the market which still use air-bleed carburetors. These offer a wide range of engine speed controls from idling speed to full power. Although this system has the advantage of allowing precise adjustment of the engine operation, it can be quite tedious and frustrating to adjust if the correct procedure is not known. It is recommended that the manufacturers operating instructions be used if they are available or can be obtained. Otherwise, the procedure that is described will work for a majority of engines.

Before starting this procedure, be sure that the fuel supply is fresh, the needle valve is set to a good starting point, the glow plug is good, and that the model is secure. Every precaution must be taken to avoid injury since the engine will be changed from idle to full throttle several times.


Throttle Valve Adjustment Diagram

Step Descriptions
COMPONENT ACTION DESCRIPTION
Throttle Open Fully Use the throttle control on the transmitter to open the throttle to the full open position.
Engine Start Use normal starting procedure to start engine.
Needle Valve Set MAX RPM Close the needle valve slowly by turning clockwise until the maximum engine RPM's have been reached without killing the engine. Open the needle valve by turning counter-clockwise 1/8 turn.
Throttle Close Fully Use the throttle control on the transmitter to slowly close the throttle to the full closed position while avoiding killing the engine.
Throttle Stop Screw Set Idle RPM Find the idle position that yields the lowest possible RPM with steady and even running by using the throttle trim on the transmitter or by screwing the throttle stop screw without killing the engine.
Engine Stops or Misfires Engine quits, misfires, runs roughly, or vibrates excessively.
Needle Valve Close to Set Idle RPM Close the needle valve slowly by turning clockwise until the engine runs smoothly.
Air Bleed Screw Screw Out 1/8 Turn Open air bleed screw by turning counter-clockwise 1/8 turn.
Needle Valve Open to Set Idle RPM Open the needle valve slowly by turning clockwise until the engine runs smoothly.
Air Bleed Screw Screw In 1/8 Turn Open air bleed screw by turning clockwise 1/8 turn.


The final setting of the air bleed screw must be tested by rotating the model with the engine idling. The model is held firmly, the nose is rotated slowly upward to about 15 degrees. If the engine runs unevenly or stops, the model is rotated quickly downward to about 15 degrees. The air bleed screw should be closed by turning 1/2 turn clockwise. If the idle speed increases, the air bleed screw should be closed buy turning 1/2 turn counter-clockwise.

If idle problems persist, this could be an indication that the air bleed inlet is too small. This is a common problem with this system. The air bleed inlet can be opened to a larger diameter which usually results in improved idling and better idle control. This is done by removing the carburetor from the engine and removing the air bleed screw from the body. The size of the hole is determined by placing drills of approximate size in the hole until a tight fit is obtained. The next larger size drill is used to open the hole. Usually, opening the hole diameter by one or two drill sizes will improve the idle.