I am just throwing this out there. Since the exhaust and intake is restricted and possibly the secondary slide, I'd verify the jetting with an unrestricted model.

Hi the 3ds has for sure a high compensation system. Works like crap. Blocked it off in mongolia last year. passing 1500meters and the power drops dramaticly.

My 3ds which is the serie1, had:

Different jetting. (Bought German 3aj carb, Ebay.de)
A plastic ring to restrict the vacuum slide for opening fully.(removed)
A restricter ring in the end of the Exhaust. ( Changed exhaust, it was rusted away anyway)
A different snorkel. (Bought German 3aj airbox, Ebay.de)
A different midter section in the airbox. (Bought German 3aj airbox, Ebay.de)
Super high end gearing. ( Changed gearing )
Small oil cooler. (Was Problematic in the hot countries, Kazachstan Etc)