I was never thrilled with that model "tuned muffler"
My preferrence is the one that is front in -rear out.
6400 on the 27x10 MenzS is crankin good power tho.
One thing I found with the ZDZ - when tuning with exhaust pressure - you can run into the "overcompressed " situation that occurs with blowers on cars.
just bumping the timing back a little may fix a ragged running and or quick heating problem which reduces power.
This is simply detonation -or pre ignition (no such thing as pre detonation as it is sometimes labeled ).
I have some race gas that I am going to try -on the bench - just to see if I can bump up the cylinder pressure any more .
I won't fly this stuff - just fiddlin around.
the slight setback in timing seems to be a easy - good running fix.