Older two stroke engines tend to accumulate oily carbon (crud) in the exhaust. With the appropriate level of heat it can be cleared and power restored.
Step 1: An old gas grill will ussually do the trick.
Step 2: Your pipe must fit inside the grill, or at least the part you want to clean out.
Step 3: Keep the fuel tank as far from the heat as possible. You will need to push the grill beyond normal operating temp.
Step 4: Add a layer or two of fiberglass insulation. Paper backing should really be removed. Plastic wrapping is out of the question.
Step 5: Keep an extinguisher handy and let her rip. Full power for about two hours.
Step 6: I had outstanding results although the grill is never to be the same. If your grill is intended for food preperation you might want to use the neighbors grill while they are in Las Vegas for the weekend.