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Author Topic:   Knocking Mercury 90 4 stroke
mojipa posted 05-31-2004 07:11 PM ET (US)   Profile for mojipa   Send Email to mojipa  
My brand new outboard makes a weird knocking sound for about 3 seconds when I come from W.O.T to lowest speed. I called my dealer and they say this is normal when coming out of a high speed.

Any second opinions?



high sierra posted 05-31-2004 07:33 PM ET (US)     Profile for high sierra  Send Email to high sierra     
Your prop is making a ratcheting sound when you decellerate rapidly. Normal. high sierra
Knot at Work posted 05-31-2004 07:46 PM ET (US)     Profile for Knot at Work  Send Email to Knot at Work     
It is your Prop

dont be concerned. Just anticipate the slow down and reduce the speed gradually.

AQUANUT posted 05-31-2004 09:32 PM ET (US)     Profile for AQUANUT  Send Email to AQUANUT     
actually its not your is affected by your props weight...a stainless steel prop will cause the noise to be is actually mercurys new style gear case..and how the drive shaft is put together in it.

to avoid it....power down slowly and evenly...instead of being WOT then immediately going to nuetral


RonB posted 06-01-2004 09:50 AM ET (US)     Profile for RonB  Send Email to RonB     
If the weight of the prop can cause an audible knocking why would one use a stainless steel prop? Is the cost justifiable? The reason I ask is that I switched from my SS Vengence Prop to a Black Max Aluminum . I did this at the advice of my Mercury mechanic who said the lighter aluminum is easier on the lower end (Opti 135), both when shifting and if you hit a peice of debris. So far I cannot say he was wrong, works for me.

I keep the SS as a spare.


bsmotril posted 06-01-2004 12:02 PM ET (US)     Profile for bsmotril  Send Email to bsmotril     
While the lighter mass of an aluminum prop is easier on the gears, your lower unit on your 135 is the same gearbox used on the Opti 225. I would use whatever prop gives the boat the performance level you desire, and not worry about the lower unit as a factor. With regular maintenance, it will outlast the top end. BillS
RonB posted 06-01-2004 03:19 PM ET (US)     Profile for RonB  Send Email to RonB     
Thanks for the explanation Bill. I've always known the block on the 135 is shared between the 115, 135, 150, and 175, but I didn't know the lower unit was shared amongest the 135's larger brethren as well.

I guess it's good to know that a large part of the components in a 135 are designed to run in larger Opti's. Hopefully they are over-engineered for the 135 giving the 135 an extended life. With the way they sip gas and my marina charging $2.64 a gallon I do not think I'll be repowering for awhile.

The Black Max seems to be adequate for my use and definitely quieter when shifting when docking at my berth. I do not think she has the WOT performance of my SS prop.


Knot at Work posted 06-01-2004 03:28 PM ET (US)     Profile for Knot at Work  Send Email to Knot at Work     

Think Mojipa was the original owner of this thread I hope his question was answered.

elaelap posted 06-01-2004 03:54 PM ET (US)     Profile for elaelap  Send Email to elaelap     
When I sea-trialed a Montauk 170 earlier this year, the motor made some sort of knocking sound when I deaccelerated rapidly. Before I could ask Craig (many of you West Coast guys know him--he's been at the Outboard Motor Shop in Alameda for many years and is a very helpful and knowledgable Whaler dealer) what was going on, he told me that the sound had something to do with the way the prop was connected to the shaft, or a special Mercury spindle, or something like that. Bottom line: Craig told me it wasn't a problem with the motor and the sound could be avoided by slowing down slowly. It didn't exactly enamor me with the Merc 90 hp four stroke, truth be told, but I sure thought the 170 was a sweety!


elaelap posted 06-01-2004 06:54 PM ET (US)     Profile for elaelap  Send Email to elaelap     
By the way, as I reported in an earlier post just after I sea-trialed the 170, Craig told me that they were spec'ing a surprisingly large number of the boats with Mercury (of course) 60 (or is it 65?) hp four strokes out here. As most of you guys know, authorized Whaler dealers won't/can't spec any California BW with a two stroke motor these days, so most of the 170s get the Merc 90 4/s. Craig said that most of the demand for the smaller motors comes from commercial fishermen or guys who are going to use their boats in some other commercial fashion. I'll bet they love the fuel economy with gas prices where they are now; and it's gotta say a lot of good things about the 'Acutrac (sp)' hull design that a small four stroke can move a 1400+ boat right along in northern California's somewhat challenging weather and seas.


mojipa posted 06-01-2004 07:56 PM ET (US)     Profile for mojipa  Send Email to mojipa     
Thanks for the response on the KNOCKING issue. I notice that it knocks even when decelerating and staying in gear as well. I called the dealer and they havent heard of this problem before so I'm gonna have it looked at. Loud noises to me sound like problems. I'll let you guys know what they say.
jimh posted 06-01-2004 08:30 PM ET (US)     Profile for jimh  Send Email to jimh     
"My brand new outboard makes a weird knocking sound"

What kind of outboard is it?

Dick posted 06-01-2004 08:44 PM ET (US)     Profile for Dick  Send Email to Dick     

He states in the thread title that it is a Merc 90 4 stroke.

AQUANUT posted 06-02-2004 12:50 AM ET (US)     Profile for AQUANUT  Send Email to AQUANUT     
the mercury 70/90/115 hp fourstrokes lower gear cases will also make that ratcheting noise at engine idle , engaged in gear.

if you really wanna hear it and panic,,,run it on a flusher out of the water....noisey..yeah now you can really freak out

nature of the beast..I don't like it..but even tho I work at a boston whaler/mercury dealership...I am glad I have a 6yr warranty.."just in case"

mojipa posted 06-02-2004 01:29 AM ET (US)     Profile for mojipa  Send Email to mojipa     
So you see this a lot in this motor? Glad to know that I have nothing to worry about. Is this something you only see in the newer Mercury motors?
fitn217 posted 06-03-2004 02:37 PM ET (US)     Profile for fitn217  Send Email to fitn217     
MY 2 stroke 90 on a 03 mt170 rattles, clangs, bangs around real bad with a stainless prop on it (Laser II 20 p). The people at Mercury told me that this is clutch rattle and is NORMAL. How could something that sounds that bad be normal? On top of that, forget about sneaking up on a school of fish at idle speed. A stainless prop on my motor increases vibration and sounds as if the gear shaft is going to rip right through the lower unit casing. I'm sticking with the aluminum.
This whole experience has really soured me as I have spent $750 on two stainless props. Mercury SHOULD make a stainless prop that dosesn't sound as if it is doing more harm than good.

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