You sound as if you may still be running a tad fat. Try leaning it out a micro turn at a time. At the point you’re at on the tuning, a little adjustment will go a long way. Leave the LSN alone for the time being and concentrate on your WOT fuel to air mixture.

The grinding noise I’m not too sure about. A thorough inspection is in order on your part.

A tuned muffler is single chamber but with larger volume and with a specified outlet size to regulate a specific amount of backpressure. There is a science behind it and they work. Getting the right one is all it takes.

What percentage of nitro are you using?