merc 1994 tracker 115 knocks at 3800 rpm

[mihssen]

Hi

Anyone had a motor only knock at a certain rpm? I am unsure if it is best described as a knock, or almost seems like a "ratcheting" noise when coming back down from 4500 or so. I would almost say the sound is like something coming in and out of gear then, otherwise at constant rpm of around 3800 it is present. not there at 4500 or 2500. Any thoughts?

Thanks

 

[CharlieB]

Re: merc 1994 tracker 115 knocks at 3800 rpm

Not sure that I understand your statement.

Depending upon how fast you reduce engine speed, the prop can 'coast' through the water and you may hear the gearcase 'ratcheting' until the boat slows enough that the prop starts 'pushing' again. Perfectly normal for this to occur with relatively sudden reductions in motor speed.

Now if you are continuing to hear this noise while running at a constant speed, then more attention is need, you could be developing a problem.

 

[mihssen]

Re: merc 1994 tracker 115 knocks at 3800 rpm

Thanks for the reply. I suppose that could be what I am hearing when letting off the throttle, although I am rather slowly going from 4500 to 2500 and hear that issue, but I definately do have a knock at constant speed of 3800. Could something just be loose on the motor that vibrates only at that rpm? It goes away as I move over 4200 and if I am only at 2500 it is ok as well. Seems to be mainly at 3800. That confuses me all to heck.

Thanks again
Mark

 

[j_martin]

Re: merc 1994 tracker 115 knocks at 3800 rpm

It's probably at max timing by 3800 rpm. If the timing is slightly advanced, or the fuel is slightly subgrade, it could knock. (preignition) Carbon buildup would agravate the situation.

Try running a tank of 92 octane through it and see if it greatly lessens the knock. If it does, yer on the right track. Dog it.

hope it helps.
John

 

[brodie123]

Re: merc 1994 tracker 115 knocks at 3800 rpm

i just went thru the same problem. It was caused by an engine overheat when my impeller failed.

The knock at a certain rpm and especially as you throttle down 'can' be (not is) a bad bearing on your crank.

Take some advice and get it checked before you run her any more.

 

[mihssen]

Re: merc 1994 tracker 115 knocks at 3800 rpm

Thanks for the reply. My question would then be, why does the knock go away after 3800? I would have assumed it to be there from the 3800 and on up, but then again, I am a total novice to outboards. I run sea foam through the tank consistantly with no help.

Thanks

 

[brodie123]

Re: merc 1994 tracker 115 knocks at 3800 rpm

it will get worse as time goes on and the bearing wears into your crank