Can you provide a little more information? Did it run ok before the changes? Do you still have the stock muffler. You could add a slight amount of choke which will richen it up a bit and then see if it gets worse or improves. that will tell you rich or lean. Also look at the sparkplug is it white tan or black. Please detail exactly the steps you took to modify the motor including the order you did it in and when exactly is started acting up.
I never ran it with the stock engine, and no I don't have a stock muffler. Added all three upgrades before the initial break in, per some members' advice that it didn't matter. First time I ran it, I never went full throttle as it was around my apt. complex and tried to be safe. I took it to an empty parking lot yesterday to test full throttle and then noticed the issue. I run it with half choke as recommended by the manual.
I am looking at both the full manual and the quick start guide, The manual states that if the choke is on after a few seconds running to move the choke lever to the run position. It does not say half choke. The quickstart guide shows a picture in step 6 showing you to move choke lever from choke to run. It also does not state half choke.
