Pull the plug, read it.
I wouldn't jump to conclusions or change a thing until you look at that plug.
If there is any soot, it's rich. How about backfiring when you let OFF the throttle? That would indicate lean.
After you read the plug and think about these things, pull the carb. Has it been opened? There are three screws with break-off screws (no slot to remove). Have they been removed/replaced?
Is this a YouAll (original carb). If it is, it came with 120 main, 40 pilot jets. I installed 125 main, 42 pilot. I also shimmed the needle ~1mm. This works at 800 feet.
It might be a little too rich for 4000feet. Original jetting might do fine at that altitude.
The last thing is to drill out the airmix screw. ~1.5 turns out or up to 2 turns might work.
If PO changed the carb, you are starting over. What model, what jets?
Check out
https://www.chinariders.net/showpost...59&postcount=4
My guess is SHIM the needle, maybe a mm. That might fix it, period.