[TominMO]

Check the valve clearances. You want approximately .06 - .07 mm, or .0025" - .003". Exhaust valves always run a little hotter than intake, so if anything they should have a larger gap. Chinese bikes are notorious for having valve clearances too tight from the factory, causing overheating of the head.

Edit: Overall I agree with 4T_Goblin's post below. Intake .003"/.08 mm. Exhaust I'd go with .004"/.10 mm. These are the recommendations for Lifan's new KPX. Before the KPX they said .06 mm for both valves on the X-pect for instance, and you would often find them from the factory much tighter than that even, like .02 mm, and require immediate adjustment. Finally they are getting their act together with valve clearances. Any time you get your hands on a Chinese bike, check valve clearance so you have a baseline for troubleshooting.

IMO .005"/.13 mm might be a little too loose, since the exhaust valve will not be opening as far. That also contributes to heat buildup, and a bit of power loss due to inefficient exhaust breathing. So the trick is to find the happy medium for valve clearances. For me that would be .08 mm intake and .10 mm exhaust. However, it depends on how hot the individual bike runs, so maybe for 4T_Goblin this is proper.

AFAIK, no one in the history of the universe has ever gotten a burned intake valve, because the fuel coming in cools off the valve. But the hot expended gases exiting thru the exhaust valve heats it up, so it is more important to make sure its clearance is not too tight or too loose. If the clearance is too tight, then as the exhaust valve stem heats up and lengthens, the valve does not fully seat and you lose power. If it's too loose, then it does not open enough, and the hot gases can't fully leave the combustion chamber. Screwed either way. The intake side is less critical.

As far as the dark plug goes, your replacement carb is running too rich. There is plenty of info on here and YT for tuning carbs.