Merc '76 1150 'slow to go' issue

So I'm still learning, new to boats, new to this one, new to it all...
This motor is on a 84 Venture Bass Boat..
I was taking the boat to the 1 of 2 mechanics in town, but he seemed a little shady, and then him or someone else bent the heck out of my cowl because they ignored the fact they didn't get it in the spacer and broke that too.. I just glued it back into place and bent it back, but definitely not going there again...

So, this new place did a lync and sync like I asked, and turned down the idle rpm from 1200-1500 (the other guy set it at that!) down to 900-1000.

Motor idles well, the control box seems to operate well, shifting is waaaay nicer than the clunkity-clunk with high rpms....
But, my g/f and I sat in the boat, and I put it all the way forward.
The boat accelerated, and then didn't, sitting at about 15 mph and 2k rpms.
Out of curiosity, I told my g/f to go sit on the front (since it's at a good 30 degree angle before plane) and the boat leveled out a little and then took off (she only weighs about 140 lol), got up to about 40mph. (I had just filled the 15 gallon gas tank in the back as well)

I took it back to the mechanic and said we need to look elsewhere, originally the control box didn't match the engine, couldn't get WOT.

I mentioned the guy I bought the boat from said it needed a new distributor cap, but otherwise the mechanic noticed it was a 21 pitch prop on it. (I saw somewhere that a +/- change to prop is 100 rpm per 1"?)

I've read a bit here and there, but don't want to be too confident and would like to hear what veterans think about this issue...
Also, is 40mph a decent top speed for a 115 hp engine on a light fiberglass boat? (I thought it would be more)

Thanks very much!

Re: Merc '76 1150 'slow to go' issue

How may rpm's at wot and what pitch prop are you running ?

Re: Merc '76 1150 'slow to go' issue

I can't say I've paid attention to the WOT rpm's... since up until now the control box throttle lever didn't actually opened the carbs to WOT..
21 is the prop...

I thin I saw somewhere 5800 is max rpm rating for the motor? So 21 pitch prop is too high? (that would put max rpm's at like 4500 or such?)

Re: Merc '76 1150 'slow to go' issue

21P prop might be a bit much, but hard to tell. Try a prop with a smaller pitch and see if it is better.

The distributor caps almost never need replacing, so the mechanic is likely full of stuff. With the motor off, advance the throttle all the way and remove the wrap cowl. Look at the carb throats and see that the throttle plates are nearly wide open. I would really recommend a compression check, spark test and check the link and synch. Not sure where to go for that.