If your old 100cc motorcycle is not idling properly when it's off the center stand, despite having undergone an overhaul and other adjustments, there are a few potential factors to consider:
Carburetor Adjustment: Check the carburetor settings, including the idle mixture screw and idle speed adjustment screw. Ensure they are properly set according to the manufacturer's specifications. Incorrect carburetor adjustments can lead to idle issues. Consider cleaning or rebuilding the carburetor if it hasn't been done during the overhaul.
Air Intake and Fuel Flow: Verify that the air filter is clean and not obstructed, as a clogged air filter can affect proper airflow and idle. Additionally, check the fuel system for any blockages or restrictions that could impede the flow of fuel. Ensure the fuel lines are clean and clear, and the fuel filter is not clogged.
Vacuum Leaks: Inspect for any vacuum leaks in the intake system, such as cracked or disconnected vacuum hoses, loose intake manifold connections, or deteriorated gaskets. Vacuum leaks can disrupt the air-fuel mixture and affect idle stability.
Ignition System: Confirm that the ignition system components, including the spark plug, ignition coil, and spark plug wires, are in good condition. Faulty ignition components can lead to misfires or weak sparks, resulting in rough idle or stalling.
Compression and Valve Clearance: Ensure that the engine has proper compression within the acceptable range. Low compression can affect idle performance. Additionally, check and adjust the valve clearance to the manufacturer's specifications, as incorrect clearance can impact engine performance.
Exhaust System: Although you mentioned changing the exhaust, ensure that the new exhaust is properly installed, not leaking, and not causing any exhaust restriction or backpressure issues.
If you have performed these checks and adjustments but the issue persists, it may be beneficial to consult a qualified mechanic or motorcycle technician. They can conduct a thorough inspection and diagnose any specific issues with your motorcycle that may be causing the idle problem.