yamahaxj650

Hello I just bought a 1980 yamaha xj650 I think there is supposed to be a G after 650 not sure but I was wondering on my original side cover it says MAXIM-I. I don't know what the I stands for I guess it was inline 4 but not sure if you could help me out with any info on this bike I would be greatfull for I would like to share with others on the road and at least know a little about my bike. Thank you for who ever replies have a good day.