What are the causes of unstable idle speed caused by the carburetor?

[ValeriaFitz]

The causes of unstable idle speed caused by the carburetor include: incorrect ignition timing, excessively lean or rich combustible mixture, improper contact gap of the circuit breaker, and excessively small spark plug electrode gap. Incorrect ignition timing: When ignition occurs too early, the negative work of compression increases, hindering the low-speed operation of the engine. If incorrect ignition timing is detected, first check the contact gap of the circuit breaker and adjust it to the specified range of 0.35-0.45 mm. Excessively lean or rich combustible mixture: When the combustible mixture is too lean, it can cause difficulty in starting the engine, carburetor backfire, unstable idle speed, reduced power, and unstable engine operation. When the combustible mixture is too rich, it can cause engine overheating, insufficient power, exhaust pipe popping, slow acceleration, and unstable idle speed. It is necessary to check and adjust the fuel level height in the carburetor float chamber. Improper contact gap of the circuit breaker: If the gap is too small, under low-speed operation, the resistance decreases relatively, resulting in weak or intermittent sparks, which cannot properly cut off the low-voltage current, thereby affecting the strength of the high-voltage spark, leading to misfiring at low speeds and causing unstable engine idle speed. When adjusting the contact gap of the circuit breaker, remove the distributor and distributor rotor, turn the crankshaft to fully open the contacts, loosen the fixing screw of the fixed contact, adjust the eccentric screw, and use a feeler gauge to measure the required gap, then tighten the fixing screw of the fixed contact. Excessively small spark plug electrode gap: If the electrode gap is too small, the arc energy increases, but because the flame kernel is close to the electrode, the flame-quenching effect is obvious, causing incomplete combustion of the mixture and resulting in unstable idle speed. Generally, the gap should be between 0.7 mm and 1.3 mm. If the gap is too small, it can be adjusted with a screwdriver; if the gap is too large, gently tap the side electrode with a small hammer. A spark plug gap gauge must be available.