[wanab65]

You mentioned you had the stock side pipes. I had the stock side pipes on my 1991 CR and found the motor hanging back at higher RPM (351w). After reading several comments, by those who had already experienced this same problem, I found the 4 inch side pipe actually had only about a 1 inch pass through that was off set from the center line. I replaced the muffler with a more open center line pass through and the difference was very noticeable. Also consider ceramic coating the headers and collectors. I did and the headers and collectors run 110 degrees cooler than the muffler. There is a ceramic coating that comes out looking just like chrome.