1996 115 OB Knocking

[mustanghorizon]

Hi all,

I have a 1996 115 HP Johnson OB (Model J115SLEDR) on a FourWinns Horizon. Recently the motor has been making a knocking noise with rapid deceleration. I've read through the forums to find that it could be as simple as exhaust resonance. But today to test the motor more, I ran the motor in neutral. The motor begins to knock in neutral when it reaches about 3000-3500 RPM. Anything lower or higher the motor runs fine. I'm assuming that there is no lower motor involvement since the knocking happens in neutral. The noise is definitely coming from one of the drivers side cylinders. The boat shifts fine and drives great when accelerating, no knocking or hesitation. I am thinking the worst right now. Are there any steps to be taken to deduct what could be happening. I'm hesitant to bring it to a mechanic if there is something I can do on my own. Any ideas would be more than appreciated.

 

[racerone]

In nuetral the gears and bearings are all turning.----So drain the gear oil and look at the magnet.-----Easy to pull the cylinder head to have a look in there.---Will cost a few bucks for new o-rings.

 

[bob johnson]

the word is not to rev a motor more than about 2000 rpm, in neutral......especially 3500 rpm!!!!

it might be pistol slap.....I had that in a 1997 90 hp ocean runner....motor ran good otherwise....but it was only going to get worse... on disassembly the pistol skirt on that cylinder was taking a beating.. and the bore wall wasn't the best either.....I think it might have over heated and the piston warped slightly( I bought it used like that)

good luck

bob

 

[mustanghorizon]

racerone- I will definitely drain the lower end, although doubtful that is the issue. It is definitely more of a hard knock, as opposed to the rattling often found in bad lower units. Still worth a shot though. Is there a way to check pistons without taking the cylinder heads off?

bob- I know it's not good, but I had to test it! I was hopeful that it was piston slap, but it is way too damn loud. Thank you!

I am leaning toward a bad piston rod, but it's obviously hard to tell without opening the motor up. Would it be bad to keep running the motor in the meantime? Also, would a compression test tell me anything useful?

 

[racerone]

A compression test will be usefull..------Post your numbers.--------Pulling the cylinderhead is simple and you may only need to pull one after results of the compression test.-----A bad rod on the big end is expensive !---------If you keep running with a bad rod bearing it could lead to a " dynamic halt " and then the search for a new motor.