Hi Ken, I don´t know if this would be your problem or not but years ago when I worked in a Motorbike shop when the new Yamaha´s came in we removed the exhaust pipes and ground the welds off the restrictors in the pipes. If you haven´t sorted your problem yet it might bear looking at. It´d just a piece of pipe necked down to reduce the size of your exhaust and tack welded in the pipe. Cheers Ken