Well black smoke is usually a sign of richness, however since you do not have any mods I do not think it would be jetting? Did you put the 142.5 jet in? That seems higher than what I remember the stock jet being, and I do not think you would have to rejet for just a K&N filter. Try to run it without the air box cover on. This should lean it up and if it runs better you know your 142.5 is to much for it.

 

The Black smoke you are seeing is from carbon build-up. Probably from where you cant rev it up. I would try a new plug too. Take the carb all the way apart and see if there is anything in the main jet curcuit and in the needle area. Also clean out the entire float bowl. Sounds like something is getting plugged up. If that doesnt work, clean out the tank and petcock. Then try new gas. One of those things should fix it for sure.

 

Remove the four screws from the top of the carb. Lift out the rubber diaphragm and check the small plastic disc that sits on top of the main jet needle (under the large spring) to make sure it is in place. It can be off to the side and allow the needle to raise up causing the engine to run rich.

 

I also have a k&n filter.... the seal on the front of it came loose and worked up into the air passage where the rubber intake fir onto the filter box. same problem. 3 screws removes box(with filter in place) for inspection. look inward from hole to carb, check gasket for proper fit, fuel disolves glue holding seal onto filter.

 

Check the vent tube for a clog if it is not free flowing the float will stay open and flood your machine this is usually run up under the fuel tank along the frame.

 

I'm with trailcrawler on this one.
I think you have some type of air intake obstruction.
Just take the filter out and look towards the carb.
If all looks good there take it for a short test ride with no filter and see how it acts.