I've never tried this on a pipe, but it works on rusty/dirty steel gas tanks: Pour in about a 1/4 gallon of Diesel and some of that clean white chat rock like they use in gardens. You can get the rock from Home Depot or any place with a garden center. Seal both ends. Duct tape works, or make plugs from a big ball of duct tape, or even a rag. Make sure the chat-rock is clean before you use it. Take a torch or Map gas to the outside of the pipe and heat it all over. Shake the crap out of it for a couple minutes, let it set and cool off until you can handle it, then repeat the process 3-4 times. It also helps to twist and turn it every which-way between heat/shake cycles to distribute the diesel and rock. Pour out the rock and diesel, which you can save and reuse if you want. Heat it again, and flush it out with diesel but without the rocks, then finally blow it out with high pressure air to make sure you get out all the mix. 90% or more of the carbon will come out with the diesel and rocks. If you have a stubborn rock or two that doesn't want to come out, flush it with a garden hose, then blow it dry again with the compressed air.
Someone will, like I did, figure that gasoline or kerosene will work better and break up more carbon faster, which it will. Just remember you are using an open flame. Remember that if you use gas, you have just made a bomb of your pipe! Diesel is far less flammable. You might have to work a little harder on the shaking part with Diesel, but you'll also be more likely to survive. So will the pipe.