Idle does get better with the 18" props.

Zinger props don't do much for me. Use something else.

The plastic APC's will idle lower than a Xoar. Wood props are lighter and offer quicker gain in rpm on acceleration but they idle higher.

I have a Ryobi with a 5/16" carb and it runs just under 7000 with the old Dynathrust 18 X 8 prop. There is nothing about it that is not stock. The original muffler had a 3/4" outlet.

I wouldn't be running a Ryobi over 7500 or so because of the stamped steel connecting rod

Maybe your reed valve is seeping a little?