Engine issue or prop issue?

[jcupo6]

My 1978-1982 120hp Merc was having a stutter problem when accelerating which I have fixed. Now I am concentrating on her full throttle speed and RPM's. At WOT she runs 2,800-3,000 RPM's at about 24mph. I know the motor should be at 3,800-4,500 which would probably get me closer to 33-35mph. I am not really worried about her top speed, but if there is an issue I want it fixed. This is what has been done.

Carb cleaned and rebuilt, replaced fuel accelerator
New Pertronix electronic ignition installed
New distributor and rotor
Timing checked
New plugs and wires
All new fuel lines
New mechanical fuel pump
Compression - 1-145, 2-150, 3-150, 4-150

I have a 22 foot Aluminum Starcraft Holiday that weighs around 2000 pounds. She originally had a 140hp motor with a cracked block. I never replaced the prop when I downgraded to the 120. My question is, do you think the 120hp motor is too weak to push the boat properly, or would a better pitched prop give me the 800-1000 RPM's that I'm missing? Thanks for your help!

 

[airdvr1227]

Re: Engine issue or prop issue?

...

 

[sethjon]

Re: Engine issue or prop issue?

The engine is enough for your boat. A different prop will definitely be a plus.

 

[jcupo6]

Re: Engine issue or prop issue?

Thanks, that's what I wanted to hear!

 

[jcupo6]

Re: Engine issue or prop issue?

Update:

Installed a brand new Black Diamond 15x17" prop and instantly noticed changes! The boat now runs about 4100 rpms, up from 2800 and jumps on plane extremely fast. It's amazing how well a boat runs with a better prop! I'm still only running about 30mph, I think I should be around 33-35, but the motor is 40 years old. I'm very happy with these results.