1965 Mercury 500 50 HP issue

Hey all, been lurking around for awhile gathering ideas and advice. I have a 1965 mercury on a 1979 16' bass tracker. Motor has a new control module and plugs. Wires are in good shape, my issue is, that when on the water the motor does not run well at low RPM's. Runs like a scalded ape at WOT but stumbles over itself when in gear and trying to get to WOT. This issue is non existent when running motor on the muffs. but when in the water with realistic back pressure it stumbles to get RPM. Any advice or ideas are appreciated, on a side note fuel is fresh and ran with ethanol fuel treatment and at 40:1 ratio.
Thanks guys!

How is the compression on that motor?

compression is around 100 psi on each cylinder, all within 5psi of each other

100 # is a little on the low side but it should still run good.Sounds like it may be a little on the lean side on the idle circut.

can anyone recommend a mechanic in the wichita area?