You can use a blowtorch but be careful. BE SURE TO WORK AWAY FROM ANY FLAMMABLE STUFF! REMEMBER THAT THE BLOWTORCH CONTAINS GAS UNDER PRESSURE AND MAY CAUSE FIRE AND SKIN BURNS!
Do the following:
- disconnect the pipe and muffler from the cylinder and frame;
- if possible, detach the exhaust manifold from the muffler itself;
- secure the parts so that they don't move, or prepare some pliers with long handles to hold them;
- fire up the blowtorch, wait until it warms up and the flame turns blue and steady;
- start burning the parts by letting the fire through the pipes. Remember that if your exhaust is chrome-coated you shouldn't burn too long to save the chrome.
You will see smoke and flames coming out from them. As the smoke gets thinner and the char becomes greyish, stop burning, put the pipes on the earth and knock them gently with a wooden stick to crush the char from the inside. When all the char seems to be out, leave the pipes to cool down on the gound. Don't pour water on them - this will damage the chrome and soften the steel.
Things may become blueish from heat but it looks in a way cool, as though your ped is a kick-ass nitromethane dragracer :-)
Anyway, you may try some special chemical solvents, which is a lot safer, but that's how I tend to solve the problem.