* Yamaha never produced an XS391 motorcycle. The XS series models were the XS250, XS360, XS400, XS650, and XS750.
* 391cc is close to the displacement of the XS360. The XS360 has a 358cc engine, so the 391cc stamp could be an approximation or a slight variation.
* "4t4001999" is a Yamaha engine serial number. Yamaha engines have unique serial numbers, and this format is typical.
Here's how you can confirm:
1. Check the engine model number: Look for a plate or tag on the engine itself. It might have a model number like "XS360" or "RD350" (the XS360 and RD350 engines are very similar).
2. Look up the serial number: You can try searching online for Yamaha engine serial number databases or forums to see if you can find information about your specific engine.
While the engine displacement and serial number suggest it's from an XS360, the best way to confirm is to look for additional information on the engine itself.
