Based on the information you provided, there could be a few potential issues with your Yamaha FZs v2 motorcycle. Here are some suggestions to consider:
Carburetor Issues: The symptoms you described, such as slow acceleration and fluctuating revs, could indicate a problem with the carburetor. It may be clogged or require tuning. Consider getting it cleaned or adjusted by a qualified mechanic.
Fuel System Problems: Check the fuel lines, fuel filter, and fuel pump for any blockages or malfunctions. A restricted fuel flow can lead to poor acceleration and inconsistent revs. Ensure that the fuel tank is clean and free of contaminants.
Air Intake Issues: Inspect the air filter to ensure it is clean and not obstructed. A dirty air filter can restrict airflow and affect engine performance. Replace or clean the air filter if necessary.
Spark Plug Condition: Worn or fouled spark plugs can cause similar symptoms. Check the spark plugs for signs of wear or deposits. If they appear damaged, replace them with new ones.
Clutch Slippage: If the clutch is slipping, it can lead to a loss of power and inconsistent acceleration. Have the clutch inspected to ensure it is functioning correctly. Adjustments or replacements may be necessary.
Engine Compression: Low compression in the engine can result in poor performance. Have a compression test performed to check the condition of the engine. If compression is low, it may indicate internal engine wear and require further investigation.
It is recommended to take your motorcycle to a qualified mechanic or a Yamaha service center to diagnose and address the issue properly. They will have the necessary expertise and tools to identify the problem and perform any necessary repairs.