Suzuki dt65 88mod. strange noise

[Piten41]

Has a strange knocking noise in the engine after I serviced it.

the knocking noise follows the engine speed, but with a much lower frequency.
the knocking noise comes about once a second, at low speed (25-30 knots) and more frequent when the speed increases.
The sound seems absent when I have it in neutral and rev the engine.
No unusual noises in the prop shaft when I spin the propeller.
No play in the prop \ propeller shaft.


The only thing I have done with the boat before the noise came are: replaced impeller, shaft grease,.
No chips or water remaining in the oil I drained off.


The engine starts and spins and run just fine. running well and is quick in response.
Shift is fine without any noise, slippage of some sort.

All answers are appresiated

[keakar]

that sounds like piston slap when the engine is under load, in most cases its one of the botom two cylinders doing it.

i had 4 engines that all did it with age and it was anoying but the engines all went a few more years before breaking.

as far as i know there is no way to test or diagnose a problem like that since you arent losing compression and everything works as it should. im told it mainly happens with overheating

i would say pay attention to it and if it starts getting louder then dig into the engine, otherwise if there is no metal in the oil then just live with the tapping

[Piten41]

Suzuki dt65 88mod. strange noise

Tanks for your quick response!
if it is piston slap,should it not be much more rapid intervals?
I don`t have a rev counter, but in 25 knots i belive it about 3000 rpm, and than I can hear the noise with a one second interval.

[keakar]

piston slap can happen just at certain spots allowing the slap so its hard to say but most often its at the bottom of the stroke i think. often it wont slap every time or it would fail very quickly. what you are hearing might be a piston thats slapping only every 2 or 3 strokes. sometimes the slap comes when its under load and sometimes the slap comes when its not under load so there is no rule of thumb for it.


there is another thing to check, look under or feel under the flywheel, what you are looking for is to see if one of the magnets has come unstuck. if one was loose it holds itself in place but can move around on you as its running (and especially at startuip and stopping) until it hits something and could be "ticking" against the edge of something like a bolt or the block. its rare but it can happen