There could be several reasons why your 1978 Honda CB400a is not starting. Here are a few common issues to consider:
Battery: Check the condition of your battery. If it's old or discharged, it may not have enough power to start the motorcycle. Make sure the battery terminals are clean and securely connected. Consider charging the battery or replacing it if necessary.
Fuel System: Ensure that your motorcycle has an adequate fuel supply. Check the fuel level in the tank and make sure the fuel valve is in the correct position. If the motorcycle has been sitting idle for a while, the fuel may have become stale or clogged the carburetor. In such cases, the carburetor might need cleaning or rebuilding.
Ignition System: Verify that the ignition switch is turned on and that the kill switch, if equipped, is in the "run" position. Inspect the spark plugs to see if they are fouled or worn out. Faulty spark plugs can prevent ignition. Replace them if necessary. Additionally, check the condition of the ignition coil, spark plug wires, and other ignition components for any signs of damage or malfunction.
Starter Motor: If you hear a clicking sound when you press the starter button but the engine does not turn over, it could indicate a problem with the starter motor or the starter solenoid. Ensure that the starter motor is receiving power and that the connections are clean and tight.
Electrical System: Inspect the wiring harness and connections for any loose, damaged, or corroded wires. Faulty electrical connections can disrupt the flow of power to various components, including the ignition system and starter motor.
Other Factors: There could be additional factors specific to your motorcycle that are causing starting issues. These might include problems with the choke, air filter, or even engine compression. If you have checked the basic components mentioned above and are still experiencing difficulties, it may be necessary to consult a qualified motorcycle mechanic or Honda specialist to diagnose and resolve the issue.
Remember, troubleshooting starting issues can require a systematic approach, checking each potential problem area one by one. If you're not confident in your mechanical skills, it's advisable to seek professional assistance to avoid causing further damage.