When idling on the stand to warm up I use the thumb screw, but when I stand it upright the idle increases to the point to where I'm scrambling to undo the thumbscrew. Why does this happen?


It's an '86 Sportster 883 by the way.

 

If it still has the old Keihin butterfly carb, they were known for that. What happens is the change in fuel level from when the bike is on sidestand to upright changes the way the jets meter fuel and will affect your idle. You can get them to work OK, but if you ever get a chance to get an 88 and up cv carb, it will make you much happier. Just takes carb, manifold, cables and a $29 jet kit to make them run spot on.