I'm looking into my Poulan chainsaw (PPB4218 SAS) and the thing has very strange addition to the carb. When the throttle lever is pushed to the end it opens additional butterfly located in the channel above the carb, opening the air to the engine (bypassing the carb ???).

I believe this is what it is called "Secondary Air System". What is it for and what to do about it during conversion?

[link=http://www.lawnstation.com/Chain_Saws/Poulan/Pro/LSPPB4218.html][/link]