Okay, I'm allowed back on now...sequence of events...
1) Read somewhere on Jamis' faq that the serial number should be up to 10 digits and start with an 'H'.
2) Started looking for a Jamis serial no. decoder...no luck
3) Was pretty sure that it was asian made, as I think they used akisu forks...so, I thought I would try and find out which manufacturer made the Jamis Earth Cruisers.
4) Discovered, made by Hodaka...which I didn't know made bicycles
5) A couple searches later led me to this site...
http://www.bikeforums.net/classic-vintage/1042901-asian-serial-number-guide.html which, when I tried to copy and paste the pertinent info...crashed RRB on my computer at work due to the dreaded ISE.
6) That decoder, however, states only for pre-90...and your H1811 didn't quite fit the pattern (3rd digit should have been a letter). An '81, which this pointed to, would have had the triangle cutouts on the rear dropouts.
7) Look for other brands manufactured by Hodaka in the 90's, and check out their serial #s. Some Konas were made by Hodaka...found a reference that stated Kona serial numbers changed in 97/98 or so...so, I looked for pre-97 serial numbers and found that the first two digits do, at least, follow the 'H' (Hodaka) and '1' (year).
8) viola, 1991
Jason