76 Johnson modified 9.9 questions

[Ghostman3]

This winter I modified a 1976 Johnson 9.9 with a 15 hp carburetor. I took it out yesterday to see the difference. Before the outboard would struggle to hit 12 mph. Now it struggles to hit 14 mph. I know the 16 foot aluminum boat is heavy with a wood floor (3 guys can barely lift it), but I was hoping to hit somewhere in the 15-17 mph range. The prop is 9.25 x 10. The motor ran great (starting, idling, slow and fast), but short of putting on a 25 horse, was I overly optimistic in the speed increase?

One other curious thing was the throttle twist grip was hard to turn where it says fast. Once you applied more effort/force to get past that point it was easy to turn. It did not matter if you were increasing or decreasing speed. Once it came to the word fast on the twist grip there is a brief moment where it is hard to turn. Any ideas?

 

[muskiemike12]

Pappy will know for sure, but I think you need to do more than slap a 15 hp carb on it to get the full effect. I think you are over propped also. You should be running a 8" pitch on that rig.

 

[Charger25]

I've got a 78 15hp Evinrude. I think Mike maybe on to something with the prop. It came with an 8'' pitch prop and pushed the 16' pretty well with it loaded with gear and 2 people. But to me it ''sounded'' like it was running too many RPM's if you know what I mean.
I put a new 9.25 X 10 prop on it and ran it on the 14' last weekend, with just me and some gear for fishing. It got on plane ok and ran ok but my butt dyno tells me it not as fast and may be laboring some. I need to get a tach and check it with a GPS to be sure. What I'm getting at is if your boat is that heavy might be a good idea to go to an 8'' pitch prop. just my 2 cents