If the far right piston on your 1978 Honda CB400F is having trouble starting but runs fine once it does, there could be a few potential causes. Here are a couple of possibilities to consider:
Ignition System: The issue might be related to the ignition system. Check the spark plug and ignition coil for the far right cylinder. Ensure that the spark plug is in good condition and properly gapped. If the spark plug is fouled or worn, it may cause difficulty in starting. The ignition coil could also be weak or malfunctioning, resulting in inconsistent spark production.
Carburetor Issues: It's possible that the carburetor for the far right cylinder is not delivering fuel properly during startup. Check for any clogged jets, blocked passages, or improper adjustment of the carburetor. Clean and inspect the carburetor thoroughly, ensuring that fuel flows freely and all components are in good condition. Consider synchronizing the carburetors if they are out of balance.
Compression: Another potential issue could be low compression in the far right cylinder. This could be due to worn piston rings or a leaking valve. A compression test can help determine if the compression levels are within the manufacturer's specifications. If the compression is low, it may require further inspection and potential repairs.
It's worth noting that diagnosing and resolving engine issues can be complex, especially without direct access to the motorcycle or the ability to perform hands-on diagnostics. If you are not experienced in motorcycle repair, it is recommended to consult with a qualified mechanic or Honda specialist who can thoroughly examine the bike and diagnose the specific issue. They will have the expertise and tools necessary to identify and rectify the problem with the far right piston on your Honda CB400F.