Ahhh,, there we go...... Like said above, what carb (size, brand) and what is the jetting in that carb ? You are most likely either jetted rich or extremely lean..
Also i see you said no mods but most folks dont consider taking an airbox lid off as a mod but yet it is.. Stock air box, stock lid and a good clean filter ? Good new plug? /good fresh gas?
What was the reasoning for changing the carb?
If you did go to a stock Blaster carb, you will need to confirm the jetting as AWK said. Then you will also want to make sure that the carb is clean,,, Main jet, Pilot jet, //float isnt sticking /or leaking, vent tubes are clear etc...