My 2005 Z250 seems to idle fine but starts to hesitate and stall when throttle hits about 1/4 open.
The plug was changed. The old plug seemed to be sooted up so seems to be running rich. I then checked to see if fuel was getting to carb. No problem there. The air filter is fine. I added some Sea-Foam to fuel to see if this would break anything loose. So far it hasn't. So I am guessing this thing has a jetting or ignition problem.
Any suggestions on what to try next before I have to haul it to a dealer. [img]i/expressions/face-icon-small-confused.gif[/img]

 

you are definitly running too rich. Try adjusting it yourself.