I recently installed a newly rebuilt Mercruiser 502 Mag Mpi with Lightning Headers into my Mariah Z302 and now have past the 10 hour mark of the break in procedure and really am not sure if I have an issue with the motor or do I just need to swap the Bravo Three props to a lower pitch.

Here are the details.

RPM: 4000 at WOT ( I know very low but that is all I can get with the 26p props that came with the boat)
Speed at WOT: 40 mph (per speedo)
Weight: 5 adults, approx. 800lbs
Fuel: 1/2 tank
Conditions: 1'-2' slight chop
Air Temp: 90 degrees
Boat condition: Painted bottom, no growth (wish it wasn't but it does stay in the water for part of the summer.
Engine Temp: 170-175
Oil pressure: 45psi at start up and drops to between 35-40psi at 3400 rpm


Mercury web prop calculator states that I need a set of 22P props but with all the information I have on Mariah Z302's and Z300's that are running 502's in them and most of them are running the same 26p props.

Anybody any ideas?
Need more info?

No way you are going to be able to turn a 26" prop with 1.5 gears in the drive. A 26" prop is a 70mph prop if slip is correct. Thats a 55mph boat max with a 502mpi. You need a 22" prop.

i agree,,,,, find out what ratio outdrive the guys turning 26"s are running. betcha they are 2.0's

Sorry guys but I forgot to mention that important detail.

Running a Bravo Three 2.0 ratio.

that boat should run a solid 50 MPH. something is wrong.

Check the drive ratio again just to be sure.
Check the compression on the engine. I've seen plenty of rebuilds that are crap.
While you are checking the compression take a look at the plugs to make sure they all look the same.

Sounds like either you got your drive ratio wrong or you are running on 7 cylinders.

Thanks all.

I will be checking compression and plugs later today. I am also going to change the oil and inspect it's contents and hopefully will not find too much metal shavings in it. I have a Fram HP5 oil filter on it now and am going to replace it with a K&N one as after reading on this site that it may help with bringing the oil pressure up a tad bit. Any recomendations on what oil and weight to use?

I also had replaced the exhaust gasket last weekend due to a poor seal on a couple cylinders. It was probably caused from the reuse of the gasket during the installation process. When using the old gasket it created a slight tapping/puffing noise but that has been corrected with the new gasket. While the headers were pulled away from the block I did notice that all cylinders looked to have the same amount of coloration and none stood out as possible non firing cylinder.

What was in the boat before and what would it do?

Kendall 20w-50 / Wix filter

I bought the boat with a seized 7.4MPI (310hp) from Long Island, NY and was told by the previous owner it would run close to 50mph but have no way of confirming that to be correct. I have kept the original drive which has a 2.0 ratio and the 26P props and just changed the oil in the lower unit. I have spoke with other owners and they are running the 26P props and are getting into the mid 50's which is slow on this forum but plenty fast for the family. I am not sure that changing the props down to a 22P is the right thing to do if there is an issue with the engine. Everything sounds fine with the engine (no misfire or hesitation) and has tons of low end torque but the issue has to do with the low 4000rpm WOT.

I am most likely going to have to contact my local Mercruiser mechanic and have him bring his test tool out to the boat and see if any errors pop up on the ECM. Does anybody know of a good honest mechanic in the Jacksonville/St Augustine, Florida area they would recommend?

Being that it is a Bravo3 with a 2.0 ratio, then a 26" sounds like it should be correct.