Direction needed.

jspike91 · 08-19-2014, 06:26 PM

Okay guys, I ran my z258 502mpi for the first time this past weekend. Have a few statements and questions.

Boat has been sitting for at least 2 years.
Had a half a tank of fuel in her for no clue how long.
Ran her several times with water hose.
Propellers are a little dinged and dented.
They are both 28p 3 blade.
My outdrive is a 1.81r

Now, she comes to plane fairly quickly.
However, the engine seems to not be building power like it should.
I watched a video on youtube of a several boats with the same 502 mpi engine and compared it to the way mine had come up to plane.
No idea what the drive ratio or prop pitch was on the the other boats, but the engine just built power a lot quicker than mine.

Also, while in neutral, i can rev the engine to 5000+ rpm.
Under a load, I cannot get above 3800.
Thus my reason for saying that the engine is not building power.

Plans are to drain the remaining fuel out of her, new plugs, new fuel filter, and check the ignition system to see if this cures the rpm problem.

Now, is a 28p 3 blade enough for my boat, or would I better benefit with a 4/3 prop set, or a 26 3/3 set. Sorry for the long post, just trying to get set up for what i may need to do over the winter.

Thanks,
Justin

Motoxxx43 · 08-19-2014, 07:00 PM

Where did the boat run before and where are you running it now? Elevation plays a part here. Sounds like too much prop to me.

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jspike91 · 08-19-2014, 07:15 PM

Lake Norman in NC. The elevation has not changed since the boat was purchased new. I bought it from another businessman in my town who bought it from a businessman that i know in a neighboring town. it has been on the same lake since new.

Motoxxx43 · 08-19-2014, 07:16 PM

That is odd. Is it possible it never ran at its ideal WOT rpm to begin with?

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jspike91 · 08-19-2014, 07:27 PM

Have no idea. I am taking it to a mechanic next week who can scan the engine pcm, and hopefully get the fuel system cleaned up and replace the plugs. I bought ac delco plugs to go back in it. it has auto lite's in it now, which i am not a fan of. I know this is a large heavy boat, but im thinking with a rating of 400+ hp, it should push this thing fairly hard. Im sure that this thing needs to be run and that will be as good as anything for it, once it has had a good fuel and ignition cleaning. I have another reason i believe it could be in the fuel system. Sunday afternoon, we were swimming in a cove, and when we went to leave, the engine started, spit and sputtered til it cut off, like it flooded out. it spun over fine and would hit, then cut right back off, even with starting fluid. so i just opened the hatch, let her sit for a few, put the throttle about half way up, and she cranked up and ran fine the rest of the day. tore my nerves up for a few mins though as i do not have the insurance nor towing yet.

99 Mariah 258 · 08-19-2014, 07:47 PM

My 99 Z258 with a 454 has a 3/3 26p setup and gets on plane fast....The most I've seen her up to is 4000 rpm but I don't really trust the gauge.....it felt like less.....Also I think my fastest was around 50 but that's on the speedometer not GPS.....and mine is bottom painted.. I'm no mechanic but draining the tank, a good tune up and filter will go a long way......I haven't tuned up mine yet but she fires up quick always with no hesitation........knock on wood.....

jspike91 · 08-19-2014, 07:51 PM

My speedo isn't working, but my buddy said he was showing 55-60 on his new jet boat. so i figure 50 plus with no problem, but there is no way it is turning all the rpm it can. i have an old bb chevelle, and you can tell when that bb gets to turning a lot of rpm.

Motoxxx43 · 08-19-2014, 08:18 PM

Interesting read for you:
http://www.marineengine.com/boat-for...er-502-mag-mpi

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***ColoradoDiablo*** · 08-19-2014, 09:03 PM

Drain the "BAD" gas and change out the filter first.
As Moto stated need to know what you are running WOT now , then you can go from there.
Check out the "stickys" here:

Prop Talk - The Mariah Owners Club

jspike91 · 08-19-2014, 09:49 PM

Thanks to the both of you, hopefully i will resolve this problem next week. i will let you know what i find out. i really hate to do all of this work, then turn around in 2 months and put it away for the winter, but i need it right so i can concentrate on the cosmetic things this winter.