ZDZ 40 , what props are you using?
My guess-only a guess -is that your "muffler" is causing the difference .
Try this - run engine OPEN STACK- that is , nothing on the exhaust port - The engine will not shut off on idle doing this - but what you are doing is finding a base line to work from.
adjust high needle for max - NO sag rpm- measure and record rpm.
The J&A Pitts type muffler gives 6300 rpm .
(typical)on broken in 40's here -on 22x10 Zinger Pro's
I run tuned cans on my own engines which improve power and run more quietly .