Knocking in gear at low rpm

[B-Reynolds]

I have a 2002 mercury 90 elpto. I have it on my 24' pontoon boat.
When in gear forward and reverse there is a knocking or rattle that is very noticable accomponied with some vibration.
The knocking/rattle is completley eliminated when around 2000 rpm and higher.
I have recently took the boat to a tech who changed the gear oil (said it was too thin but he said no metal shavings were present) set the timing and adjusted carb linkage and cleaned them out.
When I got the boat back the same Noise was present. I took it back three times and he still could not figure it out.
Compression is 110 on all three cylinders.
I checked for any play in the pistons by removing the spark plugs and sticking a socket extension in and trying to feel for any play and the pistons were solid and had no movement.
I am thinking now maybe the clutch dogs are worn out? but I would think they would slip at every rpm not just 1000 to 1600? I first thought it could be the shift cable but i unatached it and shifted motor by hand ant the same noise is present.

Anyone have any more places to test or any ideas to fix the problem?
Thanks

 

[cell]

Re: Knocking in gear at low rpm

Does it knock in forward,and reverse gear?You mentioned vibration...Hows the prop?Is it loose,or spun?

 

[B-Reynolds]

Re: Knocking in gear at low rpm

It Does Knock in forward and in reverse. The vibration does not seem to be from the prop not loose or spun. The noise seems loudest in 2 places from one side of the motor its loud under the fly-wheel like something is rattling (rather hard). on the other it seems to be directly under the block of the motor. (where the gears meet the top unit maybe?) The thing that gets me is when the boat is above idle speed the noise is clearly gone. The motor is not loud at all so i would be able to tell if it were just the motor noise covering up the problem noise.
Would taking the flywheel off screw adjustments up??
and
How hard would it be to take things apart and check the gear teeth??

 

[Chamelion]

Re: Knocking in gear at low rpm

Mate, I've been having a similar issues of late and I've just spent a couple of hours reading every thread in the mercury forum containing 'rattle' somewhere. I know it's a long shot that you're still active on the forum, but just in case you are, did you ever get it sorted?

I've found multiple, probably about a dozen, threads all describing similar or identical issues and there is one main continually repeating fact.. NONE of the threads, and I mean NONE, were ever concluded. No answers found or just a lack of the OP updating everyone as to the outcome.. I'm unsure which.

So a shot in the dark in hoping that you got some answers that might help me.

Cheers

Matt.

 

[Chamelion]

Re: Knocking in gear at low rpm

Mate, I've been having a similar issues of late and I've just spent a couple of hours reading every thread in the mercury forum containing 'rattle' somewhere. I know it's a long shot that you're still active on the forum, but just in case you are, did you ever get it sorted?

I've found multiple, probably about a dozen, threads all describing similar or identical issues and there is one main continually repeating fact.. NONE of the threads, and I mean NONE, were ever concluded. No answers found or just a lack of the OP updating everyone as to the outcome.. I'm unsure which.

So a shot in the dark in hoping that you got some answers that might help me.

Cheers

Matt.

 

[Texasmark]

Re: Knocking in gear at low rpm

I have the same engine. I only have the knock/clunking in F gear at about idle speed obviously in the water pushing the boat. I only have it with a high pitched SS prop which is setup for a lot of thrust and is a pretty heavy prop. If I drop the pitch to 21 with a lighter prop I hear nothing.

I attribute it to "clutch dog" chatter in the lower unit. The engine being only 3 cyl and at 30 hp per cylinder at idle the power pulses aren't that close together and they are big. As a result the prop gets pulses of drive like punching a punching bag. Since the clutch dog is actually an overrun clutch with teeth similar to a saw blade, I feel that the combination causes the F gears to run back and forth over the dog and cause the noise. Obviously since you have a continuous mechanical connection up to the powerhead the noise is easily displaced.

The LU oil shows nothing irregular and I use the best Merc/Quicksilver available. Marine engine lower unit gears especially are extremely tough...I learned that on a restoration job. I have had the noise for over 5 years. I like to run the 24p prop so I will live with it. What I usually do is increase rpms till it stops. My idle rpm in gear is spec'd at 675 and I can get the noise to stop at around 1200. When loading on the trailer I shift between F and N as necessary to line up with the trailer without putting up with the noise.

HTH,
Mark