The loss of power and black smoke (too rich of mixture) was simply from the air filter plugged. These engines all run their crankcase blowby into the carb. How often do you service the air filter? Excessive blowby and oil usually is an indicator of worn rings or possibly a small hole in the piston (unlikely). Haven't been into the Vanguards, but most small engines have a sort of diaghram/baffel that catches the engine oil and returns it to the engine before it gets into the breather tube. Sometimes these get plugged up not allowing the oil to return. Look for where the breather tube exits the crankcase and see it there is something that can be unbolted at the breather tube exit. If there is, remove it and clean it out. Then, try disconnecting the breather tube where it goes into the carb and see if there is actuallyexcessive blowby and oil blowing out. If there is, the engine may be getting real tired. Also, did you accidently overfill the oil? That would sure cause this problem. Last, if the engine is just getting tired, might try increasing oil viscosity. I have used 15W-40 mixed fleet diesel oil like Shell Rotella T in my small engines for decades and they love it. Never an oil related problem. After you get everything cleaned up and maybe upgrading oil, just try running it and see if problem repeats. Hope this helps......
