Most likely it's that open exhaust pipe causing it. It isn't uncommon, when switching to open exhaust, to have the engine stutter or miss in a particular rpm range. There is a common misconception that these motors need back pressure in the exhaust system to run right. More correctly, the exhaust pressure pulses need to dampened in such a way to keep exhaust reversion pulses from upsetting the carburation. Google exhaust reversion, some interesting reading there.