You might want to listen to the Thai mechanics. That Rotax jetting chart will give you a jet that is very safe to use, but not very accurate for jetting a competitive engine. If you believe that your 158 jet is too lean, take the kart out to a track and after running a couple of laps, pull the choke lever up. If the engine runs better, you had too small of a jet in. If it gets worse, you were too rich to begin with and should go to smaller jet. You can also run the kart on the track and come into the pits running as close to full throttle as possible and shut the engine off. Check how wet the spark is and how dark the exhaust pipe is.
Your post seems to indicate your throttle is sticking open. Is slid not coming down when you take your foot of the accelerator? If the carb is not closing, that’s another problem that is a safety issue.
You mentioned that you clean the carb and that it has a 158 jet in it. Did you remove all the jets and atomizer tubes and inspect them for blockage? Besides the 158 main jet, what is the size of the other jets and atomizers and what are you float heights?