There could be quite a few causes for this, Is it a 2 stroke or 4 stroke motor,??? what brand???, is it new or old???, is it stock or did you try to adapt different exhaust, carburetor, or anything else to it that didn't come with the kit???, what gasoline are you using???, Is the choke on or off???, what is your gas/oil mix ratio if it's a two stroke???
Also pics all around the bikes as well as closeups all around the motor, etc, help immensely when trying to help you diagnose the problem.