True Mystery: 2016 Mercury 250 Pro XS - Loss of Power but still running at 5700rpms

[kevinbryant1]

I have a 2016 Mercury 250 Pro XS, <Serial# 2B125759> mounted to a 2016 Ranger Z521.

Here is what the boats doing - Cranks up and sounds fine, however, she is slow to plane. Full throttle it reaches 52mph - Fully trimmed. The speed will increase a few mph on a long stretch.

The Complete mind bender is, the RPM's still read 5700. I have been questioning my eyes, as in "did i really see 5700? did i misread?" but i don't believe i misread the rpm.

The prop is a Fury, 24p. I have not checked the coils yet, but that wouldn't account for the RPM's. The only thing that makes sense, would be a spun prop, but i did not hear anything, and 52mph seems high on a spun prop.

If you have any ideas as to why this is happening, please let me know. Any help would be appreciated, i imagine this one will take some serious thought.

[castforcash]

In my experience, a spun prop will not even get a boat on plane. Are you sure your tachometer is working correctly?

[Savage]

Is the RPM reading from Smartcraft instrument?

You do not have a "spun prop".

Have you changed the height of the jackplate?

[kevinbryant1]

I am going to tach the prop this weekend to check. And yes, the gauges are smartcraft. As for the jackplate, i have tried everything trim related while running; the best i can get is a few additional mph. Short of checking the RPM's this weekend, i am going to check the coils out.

Is there anything else you can think of, that would cause this?

Thanks for responding, i am trying my best to avoid taking her to the shop - which around here, means a guaranteed month with no boat and a roll of the dice if the problem will be resolved. Thanks again.

[JOE54]

KEV I spend a lot of time in z521 boats and yo speed is way off should run mid to high 60s easy with 24 fury/ one runs 70 light with poles and 2 people its a 2012

[JSU Fishing]

I had a similar issue with a Nitro, that on a good day, didn't lift very well with the prop I had. It seemed weak, like I could trim it high enough to get close to normal WOT, but lost some speed. Tried another prop, with no change. A friend told me to replace the plugs, even though they only had around 50 hours on them. I did, and everything returned to normal. That said, I never checked the gaps if the existing plugs, but they didn't look terrible when I pulled them.

[basshole51]

you may have a fouled spark plug...had a buddy with an Optimax and his boat ran "great" just slow holeshot and slow top end....turned out he had a fouled plug or two. Changed plugs and issue went away. Make sure you use the appropriate plugs for your Optimax.

http://www.bbcboards.net/showthread.php?t=20401

[OldTimer57]

If the RPM is right, this isn't a plug issue. Sounds more like a prop issue. IE running across a sand bar which will lower the pitch and propeller efficiency significantly. Your numbers indicate a prop slip of 33% which is way beyond reasonable. I'd check the prop first, otherwise make sure you are not dragging a sea anchor behind you.. :)

[low slung cruiser]

Prop issue...5700 Rpms is way too high for that speed...even with motor too high on jack plate

[Gamblinman]

water trapped in the hull?

[kevinbryant1]

Update: After checking the motor at the house, and becoming more confused as to why everything was checking out fine, i gave in, and took the boat to the shop saturday. Surprisingly, this shop was much quicker than i expected, and called me Monday to say they figured out the problem. The thermostat on 2 cyclinders was bad. I spoke with the mechanic, and he said 1 of the thermostats was popping at around 160+, and the other was popping around 180. Because of this, the motor was going into guardian mode. I said, so this would make the engine only run around 50mph, and the mechanic said "yes". Since this is something they can't check at the shop, i plan to take it out this weekend, and figure out if this is has solved the issue. Does this make sense to anyone? does this sound plausible?

[JOE54]

no warning horns for overheat ????? make sense NOPE 5700 is not in gaurdian

[Locknload59]

no warning horns for overheat ????? make sense NOPE 5700 is not in gaurdian

This. You will not see that kind of RPM in guardian mode.

[EuropeanAM]

This. You will not see that kind of RPM in guardian mode.

Can't really say that- as Guardian can limit you to anything from 3% to 98% available power, depending on the SEVERITY of the situation.

That being said- I sincerely doubt that the thermostats were the issue in the above scenario. If they were indeed sticking until 160-180 degrees, you would have had multiple warnings to advise you of the OVERHEATING condition.

Much more likely something like a spark plug, or even minor leading edge damage or scuffing to the 24P Fury (they can lose SIGNIFICANT lift and speed with only very minor edge damage).

[Locknload59]

Can't really say that- as Guardian can limit you to anything from 3% to 98% available power, depending on the SEVERITY of the situation.

Thank you for correcting my error Don. I had no idea that it would allow up to 98% power for some conditions.

[kevinbryant1]

I agree on the warning horn, as the horn has not been present while running - at least, i believe i would be able to hear it while running. I bought the plugs, and will change them as a backup before heading to the lake. I was reading earlier that if the battery is below 10v and above 16v, then the motor will go into guardian mode at 75% - which, 75% of 70mph, is 53 (rounded 17.5), so 53mph. I am going to test them tonight when i get home. Why would this even be a guardian mode item?

[EuropeanAM]

Engine cannot run properly with inadequate voltage.

[kevinbryant1]

Update: I checked the batteries last night, they were all 12.63. So that easy fix is out the door. I'll update again after i get the new plugs in, and test her out this weekend on the water. Thanks for everyones comments.

[castforcash]

Update: I checked the batteries last night, they were all 12.63. So that easy fix is out the door. I'll update again after i get the new plugs in, and test her out this weekend on the water. Thanks for everyones comments.

You need to have your cranking battery load tested.

[Savage]

^^^True^^^ 12.6 volts doesn't mean good battery.