Here's my suggestion I gave a person awhile back, and I know it uses "just solvents", but it works fine for me(on most occasions):
Express exhaust’s are difficult to clean, unless you have the right solvents. Here is a list of the most potent ones:
"B-12 Chemi-tool Carburetor cleaner" can be found at any auto parts store or wal-mart.
"Deep Creep Multi-use cleaner" can be found at some auto parts stores and specialty places.
And if those just won’t work, or you have really hard carbon deposits, buy some "Oven cleaner" from wal-mart or the grocery store.
A word of warning; Make sure to avoid inhaling, ingesting(kinda obvious), or getting these chemicals anywhere on you, especially the oven cleaner. I would highly recommend using long rubber gloves, some eye protection, and wear some long work pants/shirt on. And do this in a well ventilated area, preferably outside, and on a spot where you won’t mind nasty, oily grease and chemicals on.
Now that you’re ready(hopefully), try spraying the chemicals(at different times, never combine different chemicals)into both open ends of the pipe, especially the tail end, shake it around a bit, and let it soak for a little while. Then, pour the stuff out of the pipe(do this by flipping the pipe over, cycling from one of the open ends, letting it drain, then flip it again, etc), blast some water through it, and drain that out well. Then either blast out the rest of the water with compressed air, or bolt the pipe on the bike and run it for awhile. It will smoke a lot at first, but run it for like 5-10 minutes, and it should blow the rest of it out the pipe.
Just make sure you are being safe with the chemicals you’re using, and ride safely(hopefully a little faster now that you’re pipe is cleaner).
As long as you hear "put put put put" instead of a quiet "hmmmmm" sound from the tail end of you're pipe, you should be good to go. You could also try using a screwdriver to knock out some of the hard carbon deposits inside the pipe.
Hope this helps! :)suggestion