Im starting to wonder if it was in fact a seca....its an 82 Yamaha twin...has a really messed up shock set up for its rear end...the shock travels under the seat and into the bottom of the spine of the bike...ok better way to describe it is it bolts to the same point that the gas tank meets the seat.
The 82 XS400 (seca, I think) was a twin.
45 hp. Weird rear suspension setup.
SOme places called it the seca, some called it the xs400, some called it the xs400 seca.
So yeah, prolly that one.