Mercruiser 3.0 pegged at 5MPH

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dtwallace84

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I have a mercruiser 3.0 on a 18.5ft boat. I took it out the other day and when shifter was in full speed the boat was only going around 5-10MPH.
The boat was backfiring and stalling out prior to this. Anyone have any suggestions about what could be wrong?
 
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dtwallace84

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Re: Mercruiser 3.0 pegged at 5MPH

The RPMS were also running around 3000 to 3500
 
paulrfrancisco

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Re: Mercruiser 3.0 pegged at 5MPH

back firing mean the spark is firing the fuel when the intake baleves are open, so I'd guess you have a cracked distributor cap, bad or badly adjusted points, jumped time, or bad wired that are arking to somewhere else...
 
Haut Medoc

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Re: Mercruiser 3.0 pegged at 5MPH


The high RPMS is most likely a spun hub on your prop.....
Put your spare on & see if that corrects it......
#2 is most likely a fuel problem, change your water separating fuel filter & see if the condition persists......;):)​
 
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Echo720

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Re: Mercruiser 3.0 pegged at 5MPH

haut is most likely right. but i'll throw in my 2 pennies. if your motor was out of alignment you could possibly have a bad Coupler (attached to the flywheel. my old boat was out of adjustment and did that. but definitely check the prop first it would be easier than having to pull drive and motor.
 
paulrfrancisco

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Re: Mercruiser 3.0 pegged at 5MPH

I'll agree with the spun hub, but the backfireing and popping I dont know that I agree it to be water in the fuel... Usually thats just misfires, no? The backfiring, to me, is ignition related... Id say check the cap for a crack or oil/grease on the cap and wire boots at the cap... this can short the spark out to another cylender, or the wires coulod be arcing between each other.
 
FreeBeeTony

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Re: Mercruiser 3.0 pegged at 5MPH

Water in the fuel is defintely a cause for backfiring.............
 
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Robj

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Re: Mercruiser 3.0 pegged at 5MPH

I agree with the others about the spun prop. The backfiring, do a complete tune-up, it probably has not been done in a long time. While you are doing it, do a compression check also. Replace the fuel filter, plugs, cap, rotor, check the wires etc. Then see if your problem goes away. Post the results so others can comment.

Good luck

Rob.
 
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dtwallace84

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Re: Mercruiser 3.0 pegged at 5MPH

When the engine is at full throttle and seems to be running strong(after going through all the backfiring and stalling) the boat is traveling at a very slow rate.
 
Haut Medoc

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Re: Mercruiser 3.0 pegged at 5MPH


I all ready told you what you need to do.....For starters......
If your engine is not overheating, your coupler is OK.....
Change the fuel filter & see if it improves......
You could pull the dist. cap & have a look.....
You should do a full tune up, but not until you change the fuel filter & see what improvement (if any) you get......;)​
 
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dtwallace84

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Re: Mercruiser 3.0 pegged at 5MPH

Im doing it saturday so when im done doing the tune up ill post what the outcome was. Thanks for all your help:)
 
WAVENBYE2

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Re: Mercruiser 3.0 pegged at 5MPH

imported_Robj said:
I agree with the others about the spun prop. The backfiring, do a complete tune-up, it probably has not been done in a long time. While you are doing it, do a compression check also. Replace the fuel filter, plugs, cap, rotor, check the wires etc. Then see if your problem goes away. Post the results so others can comment.

Good luck

Rob.
Click to expand...

Sounds like tune-up time to me!!
 
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rndcomp

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Re: Mercruiser 3.0 pegged at 5MPH

Just so that I can learn, what is meant by a "tune-up"? Are talking about an oil change, plug change, fuel filter change, and checking the cap and rotor? Is there more to a tune-up than what is listed.

Thanks, and sorry for the dumb question....
 
WAVENBYE2

WAVENBYE2

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Re: Mercruiser 3.0 pegged at 5MPH

I also did all the routine maint. too, greased all the fittings,bearings,checked all the lube stern,trim etc...
 
paulrfrancisco

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Re: Mercruiser 3.0 pegged at 5MPH

rndcomp said:
Just so that I can learn, what is meant by a "tune-up"? ....
Click to expand...

A tune-up in general is Plugs, wires, cap, rotor, and if you have them points, adjust the timing and idle, gap the plugs, and if you have points, set the gap/dwell...
 
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ran007

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Re: Mercruiser 3.0 pegged at 5MPH

...two year old question here, but what was the outcome?

I took our boat out yesterday - '05 Searay, 3.0 Merc, and it ran like a dog. But 2 weeks prior, and for the past 4 years, its been running like a champ, topping at 45MPH. Yesterday I could only top 20MPH.

Any thoughts? I may post a new thread.
 
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