i swapped my fz600 engine for a 91 version and had to use the correct ecu.
it wouldn't start and there should was a very long (over 6month) fault finding escapade.
It was down to 1 wire wittch tested ok, but failed when electricity was used.
check your ecu number and google that, I may have one in my shed somewhere as I did the same thing.
Wasn't until i bypassed the loom was the error found