sounds like either your plug is fouled or you are running to rich air/fuel mixture try replacing the plug first. look at your old plug to determine if you are running rich or lean. try leaning or richening the mixture by changing the jet sizes and changing the clip position on your needle. But if I had to guess you are running rich...... This is just a guess!
The plug and filter are brand new. I kind of think it is lean, because it does it worse after it warms up. I have tried changing the main jet and clip position, but can't get the problem fixed.
Does it run at full throttle when open slowly?If not,check your carb float level,then check your reeds.the reeds might be worn out.Check all the basics first then maybe dive into reeds.