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502 MAG MPI lacking power

Old 06-03-2013, 05:02 PM
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Default 502 MAG MPI lacking power

I have a 1997 Stingray 230SX with 502MAG MPI with 325hrs on it with Bravo 1. Based on all that I have read and found this boat should run 72+mph. In fact I have read people hitting near 70 with 496 375HP engines. Now I understand conditions can have an effect on performance same with weight. But most recently I was only able to hit 65mph gps. This was with a 25p REV4 at 4625RPM. Boat came with a 25p Mirage + and I have hit 68 with that at about the same RPMs. But that data is not good cause I don't recal conditions exactly. I do know I had more weight though. In any case I feel I am about 7mph off from where I should be and am trying to determine the loss in power. I just did plugs, wires, cap and rotor. Engine feels strong and runs great. Idles smooth, starts right up...Mid range is strong. But I suppose when you have 415hp and are missing say even 10% power, it would still feel strong. Anyone have ideas where to look? I mean if it started hard, missed, idled like crap...I would know to look for the basics. But it doesn't. No tell tale signs. Should also note this is with tabs up (looks like paralell to hull) and trim all the way to limiter (before needing to use trailer button). I was alone, about 20-25gal fuel, about 200lbs gear. About 75* and on LOTO at about 660ft above seal level. What am I missing? Fuel filter is fresh. Fuel was 89 (was the same price as 87 so I figured wth). Prop did have a couple small dings from hitting a log earlier in the week so maybe I would lose 1mph from that. Def no more than 2. No fibrations. It did feel like it was toquing the wheel a bit as I had to steer to keep it straight (to the right). Any suggestions would be appreciated.
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Old 06-04-2013, 01:54 AM
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It does not sound like the tabs are lifting high enough.
That 25 Rev 4 is too big. You want the engine to spin up to 5000-5100rpms to make max HP. You need to drop 2" in pitch.
I doubt the problem is engine related.
87 octane will burn the most completely and make the most HP. Also, if its land fuel in MO, then it is ethanol/E10, which also has about 3% less energy.
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Old 06-04-2013, 08:14 AM
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I agree with Griff. You are not that far off but your RPMs are low, and if the prop isn't labbed your loosing some there also. A lab 23p Mirage + might be the ticket, but it might start hitting the rev limiter then. The motor sounds like it is running fine. The motor could be a little weak and only a leak down would show that, but I wouldn't mess with it and just start testing props.
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Old 06-04-2013, 08:27 AM
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Simple test to see the condition of the engine is to take a compression test.
I take one every time I put in new plugs since Im already there and only adds 30 seconds to each plug.

Mercruiser manual shows 150 psi on a NEW, factory engine.

I have 130-135 psi now on freash top end. Last season I was all over the map.. 75-150 psi and the engine "felt" strong... But head gaskets were toast.

Do a compression test. They all should be pretty close to eachother. If you have the tools you can also preform a leak down test, to let you know the condition of the bottom end.
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Old 06-04-2013, 08:58 AM
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Thanks for the replies.

I have a compression tester. I will give that a shot. What about ignition coil?

Yes, the fuel was land based fuel in MO near the lake.

These are all great ideas. I didn't think about the tabs not being high enough till I mentioned thier possition. Any more feedback from others on that? If these are indeed not pulling up high enough, how do I fix that? I do not want to drill more holes in the boat.

25P Mirage+ is what the boat comes with. I wanted better plane time and mid range power and stability so I tried the REV4 which I am loving. The guys running the lower HP big blocks get the 23P Mirage+ out of the box.
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Old 06-04-2013, 09:44 AM
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Check those tabs again, they should go higher than parallel to the bottom of the boat, and they probably do. A 4 blade prop is going to run 2-3mph slower than the 3, but if you want to run the REV4 you can send it in to get it lab finish and maybe cupped to raise the bow more if needed. That will help get your RPMs where they should be. Do the compression test and see what that shows, the head gaskets or valves can be leaking some, but it will still run fine and strong. BBlades can lab that prop.
Boat Propellers - Lab Finishing, Mercury, Hering Propellers and More!
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Old 06-04-2013, 11:06 AM
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Looking on bblades slip calculator I was at 11.23%. So I may also be over trimming. The Mirage + would blow out at full trim, the REV4 does not. So I may be pushing the trim a bit.

I will measure the tabs. I found the instal docs on Bennett's site.
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Old 06-05-2013, 01:34 AM
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Over trimming would cause higher propslip. Your propslip % is actually pretty low for a V bottom.
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Old 06-05-2013, 08:35 AM
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I was having about 6% with the Mirage+.

I looked at my tabs last night. They are mounted with a strake in the middle so not a traditional flush mount. And I do not see another way to mount them flush. They would either need to be practically next to the drive which would hurt stability or pushing them out would leave me no place to mount the actuator due to the transom design.
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Old 06-05-2013, 09:35 AM
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11% is not bad, but it is not going to be as good as the Mirage+ based on your numbers. You can get better if you lab the 4 blade, but it probably still won't be better than the mirage+. Do the other boats you are comparing yours to have the same trim tabs? My guess is that the strakes are there to help handling, but with a 4 blade prop, it is just more drag in the water
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