The below is right from BVM Jets website.......

NOTE - Aluminum Tank Cap Fittings purchased during 2003 have rubber stoppers that can deteriorate due to additives in the gasoline from some states such as California and Indiana.

The black rubber stoppers purchased from BVM post January '04 are ok. Of course, BVM will replace any problem stoppers at no charge.

NOTE - There is no problem with any black stoppers using Jet-A or Kerosene fuels.
Are you sure you're getting kerosene?

Dave