I bought a (slightly)used 350 Bruin.2005 I think.. It runs very well with an exception that it idles fine sometimes but will die without warning and has a slight pop back into the air box at random or when the throttle is slightly pressed or choked just above idle rpm. After it reaches normal temps it will idle fine for a few minutes then it will start dieing again at idle. It runs very smooth and with plenty of power throughout the RPM band. Just has the problem at idle. I filled the tank and added 3-5 ounces of B12 ChemTool to help clean the carb but I only managed to burn half of the fuel from the tank after a long day of mud and trail riding with no obvious changes. Also is there a MFG stamp or plate that shows the year anywhere?

 Any tips/help would be greatly appreciated.