its running weak !which means there is a fuel shortage for some reason whcih is why when you open the throttle and give it lots of air to burn its dies as it want more fuel too.i would check the carb jets and clean them out and alos make sure there is no water in the fuel as this will cause that too,try taking the lide out of the carb and moving the small circlip on the needle to the lowest notch(one nearest the pointed end)it will lift out of the needle jet earlier and further too.adam trained motorcycle mech.20 years in the trade.
adam bike doctor
October 2009
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