The procedure to use a jumper in the SCS connector comes from the Helm manual (factory shop manual). But I don't have a Helm book for 1990...

The idea is to tell the ECU to QUIT making it's constant little timing adjustments based on idle control and knock sensor. However...
1) I don't think a '90 Accord has a knock sensor.
2) Maybe the procedure is different for '90?
3) Maybe your base idle is set too high & the ECU is trying to pull it back?