Maxed out RPM at 6k 34mph?

[Ozer]

Hi all, first post here.
1997 maxum 2300 sc 5.7 merc 350 rebuilt top to bottom by previous owner. It was serviced by a marina from open to close of season, anything it needed was done by the owner.

Just dropped it in the water and took the first ride yesterday. Wide open rpm was at max, which was 6k and gps showed 34-35mph (speedo doesn't work)
The prop didn't have any markings on it so I'm assuming it's the original, it's a bit rough shape but no pieces missing or dented.

6k rpm seems high and concerning to me, what are your thoughts? I thought this boat would do 40s at least.
Bottom has paint if that makes any difference.

[Phillbo]

6K sounds high. You may be under propped. I'd call the guys at Hill Marine and discuss it with them:

https://www.hillmarine.net/

[mmwjr]

That engine should be around 4400-4800 rpm. Every inch increase of pitch should drop the rpm by 250 rpm.

[yahoo82]

Not sure if you have the fuel injected 5.7 which I think goes up to 5,000 RPM, but that still wouldn’t explain getting to 6k. Plus I believe with either engine the rev limiter is usually set about 150-200 RPM. You may want to check if the tach is properly set for 8cyl (on the back).

5000 RPM pulses from a 8cyl would read 6,600 RPM if set to 6 cyl.

Other ideas ...
If it feels like the engine free spins coming out of the hole then check for spin prop hub.

Also from experience, an aluminum prop can lose strength over time, effectively losing pitch when pushed hard so if you have aluminum plus a tach problem you may have poor performance at the correct RPMs.


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[Ozer]

Thanks guys, turn of events yesterday..
Went out, coming out of the marina I get on plain and after about couple minutes I smell plastic burning, look back and I see smoke coming from the engine bay..
Got towed in and took it out. Exhaust bellows were charred and boat was taking in water slowly..
Somehow engine overheated and I saw the Temps past 220 and just as I was turning it off the buzzer went off. Hopefully engine is OK and it's just the bellows..

Break Out Another Thousand lol

[jparsons121]

Check your raw water impeller. Sounds like you didn’t have enough cooling water flow. This would explain it not only overheated the engine as well as burned up the rubber bits on the exhaust. Were rpm were you running at the time?

[Ozer]

Originally Posted by jparsons121

Check your raw water impeller. Sounds like you didn’t have enough cooling water flow. This would explain it not only overheated the engine as well as burned up the rubber bits on the exhaust. Were rpm were you running at the time?

30mph at 5500 but according to the mechanic that gauge may be wrong. From the sound of the engine it's not over revving

[mmwjr]

Originally Posted by Ozer

Thanks guys, turn of events yesterday..
Went out, coming out of the marina I get on plain and after about couple minutes I smell plastic burning, look back and I see smoke coming from the engine bay..
Got towed in and took it out. Exhaust bellows were charred and boat was taking in water slowly..
Somehow engine overheated and I saw the Temps past 220 and just as I was turning it off the buzzer went off. Hopefully engine is OK and it's just the bellows..

Break Out Another Thousand lol

Do you mean the exhaust rubber hoses to the y-pipe? Sign if no water flow as it gets mixed in the exhaust stream by the elbow/riser to cool the exhaust gases. Bad impeller.

[shrew]

Originally Posted by yahoo82

Not sure if you have the fuel injected 5.7 which I think goes up to 5,000 RPM,

5.7L/350cid: 4400-4800

6.2L: 5000-5400

[Ozer]

No it's the carbed.
Its probably the impeller which then caused the overheating issue and belows to melt..
Boat is still where I parked it so they haven't even gotten to it yet. I'll update in what it needs and happens to it.