Your photo shows the smaller 180/210 Bisson muffler. The high volume muffler has a larger can.

If you can't get it to richen up BELOW 6400, it sounds like your low speed adjustment is too lean. The low speed disk is very sensitive and it is sometimes a little difficult to get it right, but once it is, it will stay right for a long time.

If your low speed is too lean and you are compensating for it with the high speed needle, that could explain your problem.

I'd suggest you find an experienced MOKI tuner to help you set it up the first time.

Jim