I have a 1980 Sachs Prima 5 505/1D with a chambered pipe. When I go to wide open throttle it accelerates then changes pitch and slows down to about 26 Recording below. But when I am at 3/4 throttle is accelerates to 31 then changes pitch and slows to around 29 does anyone have any idea on why or how to fix it.
Recording-https://drive.google.com/file/d/1RIIxozs8MSIWs1g7hsL6Yh_a3CiqJBQQ/view?usp=sharing