If your old 100cc motorcycle is not idling properly when it's off the center stand, even after overhauling it, adjusting the timing, and changing the exhaust, there could be a few potential causes to consider:
Carburetor Issues: The carburetor plays a crucial role in the idle operation of the engine. If it is not adjusted properly, clogged, or has worn-out components, it can lead to an unstable or non-existent idle. Check the carburetor for any blockages, ensure that the idle mixture and idle speed screws are properly adjusted, and consider cleaning or rebuilding the carburetor if necessary.
Air Intake System: A malfunctioning or improperly installed air filter can impact the air-fuel mixture and disrupt the idle. Make sure the air filter is clean, correctly installed, and not causing any air leaks. Additionally, inspect the intake manifold for any cracks or leaks that could affect the engine's idle performance.
Vacuum Leaks: Vacuum leaks can occur in various areas, such as the intake manifold, carburetor gaskets, or hoses. These leaks can disrupt the air-fuel mixture and result in an unstable idle. Inspect the vacuum lines, gaskets, and connections to ensure they are properly sealed and not leaking.
Ignition System: While you mentioned adjusting the timing, there could still be issues with the ignition system that affect the idle. Check the spark plug condition, ignition coil, and spark plug wires to ensure they are in good working order. Faulty ignition components can cause misfiring or poor idle quality.
Compression Issues: Low compression can lead to difficulty in idling or starting the engine. Ensure that the engine's compression is within the recommended specifications by conducting a compression test. If the compression is low, it may indicate worn piston rings or cylinder walls that require further inspection and potential repair.
It's worth noting that diagnosing specific issues without physically inspecting the motorcycle can be challenging. If you're unable to identify the cause of the idle issue or if you're uncomfortable performing the troubleshooting yourself, it's recommended to consult a qualified mechanic or motorcycle technician who can inspect the bike directly and provide a proper diagnosis.