Have you tried a different prop on it?
If you have anyone around with a different pitched prop that might help you with the bigger load.
I have a 40 horse Tohatsu 3-cylinder on my 1648 Weldbilt Modified V that I am in need of getting rebuilt but have had a hard time finding someone affordable to do it.
Everyone I have talked to about a new 40 horse have all said it would not run as fast as it did when running well.
"...one Nation under God, indivisible, with liberty and justice for all."
2016 Ranger RT188 Charcoal Metallic Dual Console
2017 Yamaha 115 VMAX SHO (VF115LA) SS Prop