Why is the motorcycle idle unstable?

[MayaAnn]

Motorcycle idle instability is due to the carburetor not being cleaned for a long time. Here is the relevant introduction about the carburetor: Definition: The carburetor is a mechanical device that mixes a certain proportion of gasoline and air under the vacuum effect generated by the engine operation. Working principle: The carburetor is essentially a tube with an adjustable plate called a throttle plate in the middle, which controls the airflow through the tube. There is a constriction in the tube called a Venturi, where a vacuum is formed. This constriction has holes that use the vacuum to draw in fuel.

[MarleyMarie]

Unstable motorcycle idle is really annoying, a common situation is the RPM fluctuating up and down just after starting. I've encountered this several times, usually due to excessive carbon buildup in the throttle body or a clogged air filter affecting air intake. If spark plugs haven't been replaced after tens of thousands of kilometers, electrode erosion or heavy carbon deposits can cause poor ignition. Carburetor bikes are more prone to this issue when the fuel system gets dirty, and worn needles can lead to inaccurate fuel delivery. Also, valve clearance that hasn't been adjusted for too long shows particularly noticeable differences between cold and hot engine states. Last time my bike acted up like this, I spent half a day troubleshooting only to find the idle adjustment screw had vibrated loose - a gentle twist fixed it. Remember to regularly clean the air filter and throttle body, and occasionally add some fuel system cleaner while running at high speeds to prevent this problem.

[StLeilani]

Unstable idle speed is mostly related to the three major systems: fuel, electrical, and mechanical. For example, the unstable quality of gasoline at gas stations nowadays, with excessive moisture and impurities, can clog the fuel lines and lead to insufficient fuel supply. Many people in our group have reported that idle speed shaking is caused by dirty fuel filters, and replacing the filter for just a few dozen dollars can solve the problem. For the electrical system, focus on poor contact of the spark plug caps or aging ignition coils, which can be diagnosed by measuring resistance with a multimeter. For fuel-injected vehicles, pay attention to the throttle position sensor (TPS) being misaligned, as inaccurate readings can cause the ECU to send incorrect commands. Mechanically, the main issues are valve leakage or insufficient cylinder pressure, which can be detected by listening for a puffing sound from the exhaust pipe. Don't skimp on maintenance costs; replace worn seals and gaskets promptly, as aging of these small components can cause more trouble than just idle speed issues.

[DiBrady]

Engine idle instability can be categorized into several scenarios for troubleshooting. If the RPM fluctuates erratically after a cold start, it's most likely due to incorrect air-fuel mixture concentration—adjust the air screw on the carburetor or clean the throttle body for fuel-injected engines. For unstable RPM when the engine is warm, prioritize checking the spark plugs and high-tension wires, especially after riding in rainy conditions when electrical leakage is common. If the RPM stabilizes during acceleration but shakes when releasing the throttle, consider overly tight valve clearance or camshaft wear. If the overflow tube at the carburetor bottom leaks fuel, replace the entire seal kit directly. For DIY repairs, start with simple steps: install a new air filter, clean the spark plugs, and adjust the idle screw by half a turn. If the issue persists, proceed with a thorough cleaning of the throttle body, ensuring the seal is properly aligned with the groove during reassembly.