How to Adjust RC Airplane Engines

Start engine according to manufacturer's instructions and bring it to normal operating temperature. Measure the engine speed using the optical tachometer. Point the sensor of the tachometer at the spinning propeller while your helper or assistant holds the tail of the aircraft. Divide the reading on the tachometer by the number of blades on your propeller to get a true RPM reading. Adjust the idle speed to manufacturer's specifications by turning the idle screw with a 1/8-inch flat tip screwdriver. The idle screw is normally located at the top front of the engine carburetor near the air inlet. Make all adjustments to the engine from the rear of the propeller. Use extreme caution when working around a spinning propeller.

Bring the engine up to full throttle. Adjust the mixture screw until the engine is running at the maximum RPM as indicated on the optical tachometer. Turn the mixture screw counterclockwise to enrichen the mixture until the engine RPM reads just below the maximum. The mixture screw is normally located on the side of the carburetor body near the air inlet.

Return the engine throttle to idle and recheck the engine speed with the optical tachometer. Adjust the idle speed to manufacturer's specifications if required.

Open and close the engine throttle a few times and listen for any engine hesitation or surges. Turn the mixture screw a little at a time to achieve the smoothest engine operation. To verify correct engine mixture, pinch the fuel line with the engine at idle. If the engine speeds up steadily before dying out, then turn the mixture screw counter clockwise to lean the mixture out. If the engine dies immediately then turn the mixture screw clockwise to enrich the fuel mixture. The adjustment is correct if the engine runs properly for a few seconds then suddenly speeds up and dies.