The service data sheet I looked at for the '82 IT465J for the Mikuni VM38SS carb showed a float leve at 27mm +/- 1mm.
However, on another site, I saw the following listed as the stock settings for the VM38SS carb
Float level: 10.9-11.9 mm
Needle clip: 2nd groove
Jet needle: 6FJ40
Main jet: 320
Needle jet: R-0
Pilot jet: 50
Slide cutaway: 4.0
Maybe they are both measuring from different points. I don't know.
IT465 Service Data Sheet , from South Seattle Yamaha Sports Plaza parts page on 82 IT465.
The Mikuni VM Super Tuning Manual
http://www.mikuni.com/pdf/vmmanual.pdf shows the float height at 22-24 mm and gives a diagram in Chapter 5 on how the measure it.
You may have different problems, but I've experienced the runaway effect before and eventually found it was a kinked throttle cable not allowing the slide to fully close.