Yanmar 3 GM (F), low rpms , top speed of 3.7 kts

[hugosalt]

If it helps, I have the same motor. 22.5 hp 3gm but it is raw water cooled.
Boat is a 30 fter. 8500 lbs. boat new came with a 14" prop with 9 pitch.
If I remember correctly, would go 6 plus knots at 2800 rpms in flat water.
(I have since installed a folding prop)
After you install your new SS mixing elbow, if no joy, would look into
your prop size/pitch. (prop seems smallish)
Last year I installed one of Ben Cottons SS elbows removing the original
elbow after 33 years.
I also removed the exhaust manifold and removed a good bit of carbon build up
which had not effected performance yet but could be a factor in your case.
Hope this helps.