What will 91 octane do for my 1983 650 hawk that 87 can't?
Well, the bike is tuned for 87 octane, and it runs well. If you
use 91 or better, it really won't go any faster, just that the
engine will run cleaner, and possibly smoother. If you want more
performance, then you would have to adjust the timing. BUT, with
the price of fuel, and if you drive a lot, better just to leave it
as is. A tank here or there of 91 is good to do though.