For a 1978 Honda CB400 with a carbureted engine, it is generally recommended to use unleaded gasoline with an octane rating of 91 or higher. The specific fuel requirements for your motorcycle may vary depending on factors such as altitude, climate, and any modifications made to the engine.
Using unleaded gasoline helps prevent damage to the engine and exhaust system, as leaded fuel can lead to fouled spark plugs, increased wear on valve seats, and increased emissions. It's important to note that leaded gasoline is no longer widely available and is not suitable for use in most modern motorcycles.
Additionally, it's advisable to avoid using ethanol-blended fuels with a high ethanol content, such as E85, as they can cause problems in carbureted engines. Ethanol can absorb water and lead to issues like fuel system corrosion, deteriorated rubber components, and poor engine performance.
When choosing gasoline, it's a good practice to check the owner's manual or consult with a Honda dealer or experienced motorcycle mechanic familiar with vintage Honda models. They can provide specific recommendations based on the unique characteristics of your 1978 Honda CB400 and offer guidance on fuel choices that would be most suitable for optimal performance and longevity of the engine.