Take a look at the carbs and see how clean they are, but usually there is no problem unless the bike has been sitting for a long time or if junk from the tank made it past the fuel filter. If the bowls look real clean, I suspect the carbs are just fine.

Take a look at your exhaust valves and see if they move with the bike running. Maybe there is a problem. I would suggest removing the valves from the heads and cleaning the carbon from the valvles and the exhaust ports.

As for cleaning the pipes, I have heard of people hot-tanking them at an engine shop. Baking also works awesome, but makes a smoky mess, so you need good ventilation. If you baked the carbon out of the pipes and cut the restrictors at the inlets, it would most likely make a DRAMATIC difference on your bike with 20,000km.
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Paul Herr
'88 FZR4/GSXR/YZF Frankenbike
MY BIKE PICS