it sounds like blow back. generally the cause of oil blow back through the carb is an after market cam, or in appropriate exhaust. in the event of an aftermarket cam, or even an exceptionally worn cam the intake valves are closing too late and when the engine fires its forcing oil, fuel mixture back through the carb. the problem is likely NOT to be your carberator. at this point since you get bad lugging I would look at the canm first and check all your lobes to see if it needs replacing, especially since you are getting soot out the backside. I would also (if your going to dig in and look at the cam) check thye cylinders sleeves and rings to make sure your not getting excessive oil bypass from your piston rings.
Poorly designed or inappropriate exhaust systems can also increase the amount of blow back, sometimes enormously. The worst offenders are overly long, open straight pipes.
I hope this helps.
all engines have a little blowback at various rpms but most of the blowback generally gets trapped in the breather and finds its way back through to getting burnt at higher rpm.