ORIGINAL: ShoestringRacer
Try closing off one of the exhaust holes on the pitts about half way and see if it improves. Close it more if necessary, you may need to close one hole almost all the way on the pitts muffler. Many people have had to do that, you can squeeze it in a vice or heat it some and squeeze with pliers.
Wouldn't it be cool if they sold a valve that you could adjust that would fit inside one of the exhaust holes. You could tweak it until it was perfect back pressure. Of course it would have to be cheap and weigh nothing.