87 yamaha 115 knocking

greatflake

Recruit
Joined
Jun 8, 2007
Messages
3
Last night i put in to do some night fishing .. running up river i noticed the motor sounded different.. turned around and stopped motor.. it was getting dark..had a slight knocking..eased back towards ramp had to run the motor was easy on it but i was still 6 miles or so out.. it got louder...trolled the last 2 miles or so.. no power loss or anything.. motor ran great last 3 times it was out.. 1 pulling tube and 2 fishing trips during the past 2 week..

I did some reading on past article and came across one where someone had a knock and rodbolt had explained to put pistons at TDC and move 30 degrees and listen and feel for movent when pushing on piston .. noticed #3 had some movement...

I've got to get a manual and compression gauge..to recheck compression
it was 120# across the board when i had carbs redone 1 month ago..

what can i expect when i start to tear down ?.. i'm pretty mechanically inclined on car engines and so just never had to work on an outboard.
is it possible to just replace that particular bearing and check the crank for any kind of damage? i'd like to get out as cheap as possible..but still have it be dependable.. kind of a cliche
 

greatflake

Recruit
Joined
Jun 8, 2007
Messages
3
Re: 87 yamaha 115 knocking

I've got a couple questions more..
what could have caused this to happen so as i dont fix it and not fix the cause and just have it happen again maybe worse? could pulling a tube with it? Its got a 19 pitch prop on it..always no trouble out of the whole but could we be putting to much stress on the motor and need to go down in pitch.. i'd be more than happy to trade off some top end when i fish if i know that its not going to kill the motor when we do watersports
 

Ray Neudecker

Lieutenant Commander
Joined
May 25, 2004
Messages
1,656
Re: 87 yamaha 115 knocking

The knocking is often caused by wear on the piston pin itself as well as the piston. Sounds like it is time to refresh the motor.
 

up3fan

Cadet
Joined
Jun 11, 2007
Messages
13
Re: 87 yamaha 115 knocking

You might want to look at the oil in the lower unit, a bad lower unit can really fool you with some knocking noises, check to see if there is metal particles or a burnt smell before you tear the engine apart!
 

Ray Neudecker

Lieutenant Commander
Joined
May 25, 2004
Messages
1,656
Re: 87 yamaha 115 knocking

Just remember the engine is 20 years old and even a Yamaha can have wear after that long a time.Rodbolt's test is very accurate. Rebuild it right and it may go another 20. Obviously all of the pistons ,rings and pins as well as pin bearings have an equal amount of wear or very close to it.
Get a factory service manual and many people are capable of rebuilding.
One of these days justintime will get out of mechanic school and go to work servicing these units and dealing with customers. He will only try his supposed solution once.
 

JUSTINTIME

Captain
Joined
Sep 2, 2006
Messages
3,284
Re: 87 yamaha 115 knocking

Just remember the engine is 20 years old and even a Yamaha can have wear after that long a time.Rodbolt's test is very accurate. Rebuild it right and it may go another 20. Obviously all of the pistons ,rings and pins as well as pin bearings have an equal amount of wear or very close to it.
Get a factory service manual and many people are capable of rebuilding.
One of these days justintime will get out of mechanic school and go to work servicing these units and dealing with customers. He will only try his supposed solution once.

ray
i have been working since may
i just give little tid bits of info on here
if i see the motor, that is a whole nother story:D
 

stokie

Recruit
Joined
Dec 16, 2009
Messages
1
Re: 87 yamaha 115 knocking

Thanks Ray for the info.
I'm looking at buying a cheap boat with a yamaha 140hp v4 motor, around 1984 year. The oil tank supply pump has done the usual fail and seized the motor, but it is running well again using manually mixed fuel, with a slight big end bearing knock at idle. Is it urgent to do any repairs, or would this motor still run for quite some time if I use plenty of oil in the mix? Thanks.
 

sdsaw

Petty Officer 2nd Class
Joined
Aug 19, 2007
Messages
188
Re: 87 yamaha 115 knocking

The knocking is often caused by wear on the piston pin itself as well as the piston. Sounds like it is time to refresh the motor.
When I bought a used boat, it came with a 1989 Yam 130 and it also came with a knock like you are describing.
Ended up being the piston pin hole in the pistons had become oblong(oval).
The motor didn't have a lot of hours on a rebuild.
The prop that was on it would only allow the motor to reach 4800 rpm.
The mechanic felt that it was over propped which lead to the piston damage.
In the future, make certain that you are not over propped
 
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