If you just rebuilt the motor and reset the carburetor to the factory settings, it might do that but you have to break it in again. Both of mine new would not run when I removed the glow ignitor for the entire breakin. After 4 tanks or so you can start leaning it out and it will run a lot better. I just rebuilt the motor in my ATX and went through the same thing. It gets better with every tank you run. Mine really started coming alive after 10 tanks or so.