As I recall, the secondary carb only kicks in after the throttle is turned a certain distance. I want to say it was about 5 degrees of grip twist?
Sadly, it's been a while since my '85 XL600. I've been enjoying following along as you breathe new life into this one!
'13 KTM 690 Enduro
'03 Aprilia SL1000
Ego reperio vestri fedei defectus turbare.