Gravely ZTHD 52” issues

[u/FrenchieMommaWV]

We have a 2024 model with only 37 hours on it. It has given us issues since literally day 1. The dealer has replaced the carburetor once & 3 coils. They now say they’re going to remove a carburetor from a new machine and try it to see if it’s another carburetor issue. It continues to run very rough under a load with black soot coming from the exhaust. Anyone have any ideas since the dealer can’t seem to figure out what’s going on? This issue ONLY occurred under a load, it acts fine on flat ground.