Before I suggest anything, let me ask you a few questions;
What is the make, model and year of this bike?
When did you first notice the problem and where there any significant things to note (How fast you were going when you noticed it? How fresh was your fuel? What octane rating was the fuel?)
Is it a 2t or 4t engine?
Is it air or liquid cooled?
I would suggest pulling your cylinder to examine the piston and ring as well as cylinder wall, but it could be something as simple as a carbon deposit blocking up your exhaust, a kinked fuel line, a blocked fuel screen or filter, a sticking float in the carburetor, a dirty or blocked air passage or filter...
There could be many things causing this problem. You will need to go through a process of elimination. Start with the simplest things first and move on.
I would suggest pulling your plug and look at it to see what color it is. You are looking for a nice even tan-ish brown color. if it looks grey, you are getting too much air and/ or not enough fuel for some reason and if it looks dark, you are getting too much fuel and/ or not enough air for some reason.
You may want to go get another plug to do a plug read diagnosis. Get the engine nice and warm with the current plug and shut it off. Pull the plug and put the new plug in. Ride it for about a minute or two, under different levels of throttle and pull that plug to check it.
My guess is that your air filter is dirty or something is sticking or blocked in the carburetor.
Black smoke usually indicates an over rich condition. Don't try to run without an air filter, because the system is very sensitive to small changes and the carb isn't tuned for it.
I know that my edger/ mulcher won't even run with a dirty filter and when it does, it smokes if the filter is dirty.
