These pipes are notorious for plugging up fairly solid, especially when people delete the autolube system. (why people do this is beyond me -- there is NO horsepower benefit whatsoever).
The autolube runs the bike at a fairly lean ratio -- over 50:1. Cuts down on the oil you have to buy & keeps everything nice & clean inside. Most people go heavy on the oil when they pre-mix, and after a few years of low RPM use, the carbon will choke that tiny opening off up by the silencer -- it can be plugged with your pinky, so it doesn't take much!
I have soaked the pipe overnight in diesel fuel, then tapped the pipe hard with a rubber mallet the next day. The diesel will soften up the carbon. The torch method is the quickest & easiest way -- as long as you have a good set of gloves!
Then you'll need to repaint the pipe afterwards.
Cleaning the pipe will sometimes wake a machine up from the dead! You'll be amazed at the power output after a clogged pipe is cleaned out.
I bought one bike off Ebay, & the seller told me he could never get it to run right. When I got it, I cleaned the carb, but it still couldn't manage more than maybe 3 MPH, full throttle in 1st gear. I noticed the exhaust was more quiet than usual, so I cleaned it out with diesel, re-installed & now it's good for 35-40 MPH wide-open in 3rd. That's the way it should be!
Cleaning Exhaust Pipe 
Linear Mode
