454 MPI rpm issue
 

I got a new to me 99 baja outlaw 25 and it came with a 454 mpi and a 21p prop. At WOT I only hit about 41mph and 4000 rpm, which seems low for this set up. I have friends with the same boat running a 23p prop running 4800 rpm in the 50's. I have changed the fuel filter as well as the ignition module (thinking it wasn't getting advanced timing) and not much change. I have the base timing set at 8 deg and thats about all you can adjust on it. I hooked up a fuel pressure gauge on the schrader valve on the top of the engine and it was about 48 at WOT. From what I'm reading, I should be closer to 4800-5000 rpm at wot. After planning, I trim up and see some gain in rpm but it maxes out around 4000. It seems like I'm getting fuel and timing, but not sure where else to check. Any help would be appreciated, thanks!

The previous owner said he had just gotten a tune up beginning of the season. New plugs, distributor cap, fluids, etc...

Yes, your WOT rpm should be 4800-5000, and it should turn the 21 easy up there unless the drive ratio has been changed from 1:50

What do the plugs look like?

Do a compression test

Is it a 7.4 mpi or a 454 mag mpi??
Did you put it into base timing mode when you set the timing??

I set the base timing at 8 deg with it in service mode

it's a 7.4 MPI

sorry Im a new member so I gotta have the 10 posts before a pic lol

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pic of plug #1
https://cimg8.ibsrv.net/gimg/www.off...47d18001fb.png

Being a 7.4 instead of 454, your WOT rpm is 4200 to 4600 and 310 HP
Don't know which plug that is but its the wrong one (extended), should be MR43T, RV12YC4, BPR6EFS

plug is ac Delco r42LTS. I’ll get the right ones and swap it thx. Will do compression test when I’m pulling them

I believe the L29 7.4’s take the LTS Delco’s, so, MR43LTS if the late Model L29. Also, NGK BPR6EFS that merc specifically lists.

Thanks for the info I’ll get the correct plugs

Parts lookip, mercruiserparts.com .
Enter your motor vin . Go to Distributor/ignition parts. :)

A 25 Outlaw with a 415hp 502mpi should run about 62-63 mph with a 23 Mirage Plus prop.
With the 7.4, you should still be running a 23 Mirage Plus but at around 4600rpm for prob 55-56 mph.
Are you possibly in a higher altitude area???

Would be good to know what the serial number is

'96 7.4 LX MPI's here and engine label says 4600-4800 rpm.

Guess the manuals need updates, neither 16 or 23 list that for MCM or MIE

I'm in dallas texas, so basically sea level. I found a couple bag plug wires and swapped them and was able to get her up to 49mph with 6 people in the boat this weekend. Im about to change the cap, rotor, wires and already have new plugs in it. Also going to service the fuel injectors. Think I'm pretty close to having her run right. Just those two wires and plugs made a huge difference! Before it felt very restricted, probably because 2 of the cylinders weren't getting proper spark. Now it feels smoother and planes much faster and less effort

Lower the pitch on your prop one click