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Mercury 150 Four Stroke has slight rough idle

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Mercury 150 Four Stroke has slight rough idle

Old 06-11-2018, 03:08 PM
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Default Mercury 150 Four Stroke has slight rough idle

Hey All,

Since this is my first outboard of this size and brand I have very little to compare to. Please only respond if you have first hand experience with a Mercury 150 four stroke. I can speculate, guess and what not on my own

I was curious about the 150 four stroke idle characteristics. This is my first 4 stroke, first mercury outboard and first outboard larger than 50 HP. In idle out of gear the motor runs absolutely smooth. In gear and a little above idle it runs completely smooth. In gear at idle it runs a little rough. Enough to rattle the boat a bit, rattle the bimini cover frame a little, and certainly feel it throughout the hull. My suspicion is this is due to the extremely low RPM for emissions standards. Occasionally it will smooth out then return to a little rough idle. Anything other than in gear at idle purs like a kitten and absolutely no starting or stalling issues. I just wanted to be sure this was normal and not a timing or mis issue. Thanks!!
Old 06-11-2018, 04:12 PM
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Sounds like prop chatter, mine does the same thing. I’ve read that a prop hub upgrade to the flo torq IV fixes it. On my boat I just bump up throttle a little and it stops, only happens at dead idle in gear.
Old 06-11-2018, 04:23 PM
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Originally Posted by Conchyjodyoos View Post
Sounds like prop chatter, mine does the same thing. I’ve read that a prop hub upgrade to the flo torq IV fixes it. On my boat I just bump up throttle a little and it stops, only happens at dead idle in gear.
that was my first thought too. U searched YouTube for prop chatter as honestly I had never heard of it until researching this motor. The video has a very distinct metallic chatter sound. Mine has no associated noise other than the engine running a little rough .Does yours make a metallic noise?
Old 06-11-2018, 04:29 PM
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Mine does the same (2017 Merc 150 4 stroke). My truck does the same due to low idle, well known. (Nissan VQ40). So I assumed it was due to low idle too. Tested that theory by bumping the throttle just a hair to increase rpm and it runs perfectly smooth. So I may not be right about the cause but bumping the throttle by 50 RPM or so solves it for me so I stopped troubleshooting.
Old 06-11-2018, 04:36 PM
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Yes, mine does make a rattling sound along with some vibration. Your symptoms seem to point to the same thing minus noise. My bet is on the prop hub. I’ve intended to upgrade mine for years but just haven’t got around to it. Maybe some other merc 150 4s owners will chime in.
Old 06-11-2018, 04:42 PM
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My 2013 Merc 4-stroke had some vibration while in gear Ann’s idle rpm. Replaced with a flo-torq IV and no more vibration/chatter.
Old 06-11-2018, 05:29 PM
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I'm following, would a lighter prop make a difference? How's your GPH at average cruise speed? Thanks,
BWP
Old 06-11-2018, 07:27 PM
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This definitely sounds like prop chatter which is apparently quite normal with a lot of engines and heavy props.
I had exactly the same issue, also with a Merc 150 Four stroke and an Enertia 19” prop. My dealer advised me when he installed the engine that it would probably happen.
An extra 50 or 100 rpm stops the chatter. I recently swapped to a Flo-torq 4 hub and the chatter was reduced by about 90%.
Old 06-11-2018, 07:27 PM
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Agree with the above, prop chatter. This is inherent with an inline 4 cylinder design running s heavier stainless prop. Get the Flo-Torq 4 hub kit and you’ll be good to go.
Old 06-11-2018, 07:49 PM
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Originally Posted by sundancekid View Post
Agree with the above, prop chatter. This is inherent with an inline 4 cylinder design running s heavier stainless prop. Get the Flo-Torq 4 hub kit and you’ll be good to go.
anybody see any long term negative wear with this condition? Mine is just a slight wobble vibration I can live with as long as I know nothing will wear at an accelerated rate down the line.

Thanks for all the reaponses . appteciate knowing my motor isn't unique.
Old 06-11-2018, 09:17 PM
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I have 5+yrs and 750+hrs on mine with no noticeable negative impacts and my gear lube is serviced regularly. With that being said I’d upgrade to the flo torq IV hub and enjoy the engine.
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Old 06-12-2018, 04:04 AM
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Try a Flo-Torq IV prop hub and report back. Prop chatter is something that most 4 cylinder outboards have to deal with, no matter the brand. Fortunately, Mercury has designed a hub to provide a remedy.
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Old 06-12-2018, 05:45 AM
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Originally Posted by Openingshok View Post
anybody see any long term negative wear with this condition? Mine is just a slight wobble vibration I can live with as long as I know nothing will wear at an accelerated rate down the line.

Thanks for all the reaponses . appteciate knowing my motor isn't unique.
No, it is just an annoyance.
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Old 06-06-2020, 04:10 PM
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I found this thread and wanted to comment. Same as OP, my Merc150 4 idles about 650-670RPM and in gear I can feel the vibration in my seat and in the boat.

I swapped out the original stainless 3 blade for an aluminum 4 blade, so I’m not turning a heavy prop by any means. I installed with a Flo-Torq II hub kit.

Runs fairly smooth at idle. Just did 100 hrs service incl oil, filter, inline fuel filter, plugs (gapped), ethanol free fuel, wiped w a diaper, etc.

Still points to prop chatter?

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