Engine changes between 89-90?
Different carb and stator.
the newer k5 engine came in 89.. and in 92 were the last motor update ; crank with more weight and they changed to statorplate with the bigger flywheel.. (the pwk 39-carb came in 88 .. 83-87 were mikuni and keihin-pe roundslide carbs installed in stock.)
the motor updates are shoulded in, that the most of the riders were going to use 500īs outside of mx-tracks for desert-races etc.
and all what i can say, only 400,- dollars is always a big deal for bikes in any conditions.