So my original motor blew and I took the plunge and bought a new 2024 Mercury 4 stroke.

Shop told me I could run same prop but I’m about 11 hours in and not getting the speed or rpm’s I was getting with the old motor. Old motor would hit 69-70 mph @ 5850 rpm’s.

New motor is turning 5650 rpm’s and getting to around 64.5 mph.

I have a 12” manual jackplate that they said I shouldn’t have to mess with either.

25P tempest 3 blade prop

Any suggestions?

Are you sure the motor is out of break in mode? It won't do anything right until then.

If it is, then what's the boat telling you?
Is it running flat? No lift? High roostertail? Really stiff steering?
You're too high on the plate.

No tail? Steering is numb?
You're too low.

If you know the height is correct, then next step is Josh at Steve's prop on Fork. That motor needs to run at the top of it's rpm range to produce horsepower, lugging it won't work at all. Prop pitch is like gears in your truck, you have to match the pitch to what your motor wants. Is that going to get you five mph? Maybe, all you can do is find your potential, whatever happens happens.

Four stroke Mercs rev to 6200 rpms. When going from a 2 stroke you need to drop 1-2" in pitch. Also need to have the throttle checked to make sure its getting 100%. That needs to be done by dealer.

They did check that it’s getting 100% throttle before I left, I remember that.

They also told me my prop would be fine but I don’t know all the specifics on what makes what does what.

It seems to drive and handle the same as the old motor rooster tail and all. Just doesn’t have the max rpm
or the speed.

Break in mode could still be a thing, it calls for 10 hours of break in but I’m way shorter than I thought I was. As of right now I’m only at 3.6 hours.

Dealer told me vary throttle til 2 hours and then drive it normally but don’t drive it WOT for long periods of time. I haven’t done that but a couple of short outbursts just to see what she would do and the results are what is concerning me.

Yeah until the computer lets it go full power you don’t know what you’ve got.

Should hit over 10 hours this weekend and will report back with results.

Thanks

Over 12 hours now and really not seeing much of a difference from above. Going to call Josh this week and see what he says

I’ve never heard of anyone running the same prop when they went to the Merc 4 stroke, most drop back down to 23’s, which is why they were so hard to find for awhile. I know you want to believe what the shop told you, but it’s apparent they told you wrong.

Here, I'll quote this again. Also, there in no break in mode in 4 stroke Mercury's. The motor will not gain rpms once you get past a certain amount of hours. The break in is a procedure on how to properly seat the rings. Also the gearcase is different and more efficient to and reduces prop slip. In almost alll cases you need to drop 1-2" in pitch when going from a 2 stroke opti to a 4 stroke. Lastly if the shop who installed it said you can run the same prop then they are a bunch of idiots and I would be suspect if they even knew how to use a computer and the software to see if you're getting 100% throttle.

I had a legend we took a prop XS 350 off and install a 250 xs 4 stroke he is running same prop and gain MPH

Well heck, I haven't called Josh yet. To be honest I feel kind of bad calling someone and asking for their expertise and not buying anything from them.

Lots of different info on this so maybe I will just try a different prop and see what happens.

Just hung up with Josh and what a great guy!!!

He asked what dealer told me that and said it wasn't right. Got a game plan on what I am going to do next and will update everyone after I try it.

Thanks again for all the help!