The European 89 model is the same as the 90-91 in the US. Page 13-4 of the Honda manual shows no spacers or washers on the bolts holding on the calipers. A cheap set of calipers can measure the distance between each fork and rotor to see if they are the same. If not, make a shim to place on the axle. On my old Suzuki, I had to make a shim by sanding down a brass washer. Are you torquing the two bolts holding down the caliper to 27 N/n (2.7 kilogram/meters, 20 foot/pounds)?

Also, the axle comes in to the wheel from the left (sitting on the bike) side. The wider spacer is on the left and the thinner on the right side. Torque on the 22mm axle bolt is 60N/n (6.0K/m, 43 foot/pounds).

 

Page 160 of the hand book 0.7MM GAP . You need to check it any time the wheel is off just nip up the fork and tap the lower for the gap. Note 87/88 3.5mm disk 89on 4mm and do fit.