Jeff,
Honda P/N 16957-ZE1-812 is still available (about $3), and I also found P/N 16955-028-004 which may be discontinued. As you know, the '78 CR250 parts diagram only shows a complete petcock assembly, so I really don't know what's inside it. The P/N's above match your description, though, and they fit a gazillion Honda models (including the '75-'77 MR175's, which is what I got mine for).
Ray
_________________ '74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric
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