A little rattle never hurt no one...hopefully..

[This_lil_fishy]

1986 Merc 50 3cyl 2stroke.

Finally got it running up to plane after finding a bad plug. A little question though, at about full throttle there is a faint random rattle sound from the engine. If I drop the throttle from full fast the rattle is very pronounced. The engine is whisper quiet from 3/4 throttle down to idle, and idles fine. I noticed another newer boat with a merc making a similar noise as it went by, so i carried on. There was no performance change the whole day...3 hours of running time. It sounds almost like a bit of pre-ignition (I will research timing later) but thought it odd that the noise was very pronounced if I came of throttle fast...and dissappear once the boat slowed to match the engine speed.

Just curious...I don't have a tach, but I think my chewed up prop might be allowing the engine to over-rev, possibly heat up to much? Once I noticed it doing this I ran no higher then 3/4 throttle the rest of the day. Did a compression test, and got 120psi (+/- 1 or 2psi) on each cylinder. No metal on the plugs...

Ian

 

[This_lil_fishy]

Re: A little rattle never hurt no one...hopefully..

I'll accept a shot in the dark guess...

 

[MadProps]

Re: A little rattle never hurt no one...hopefully..

I have the same issue with a 40hp 402 and have posted on several boards and no one seems to reply on the subject, I also see a couple others have the same issue with no repiles....sounds exactally like advanced timing ping in a 350 chevy....I cant seem to find a definative answer whether 2 strokes have a pre detonation ping aswell. possibily upper bearing is going south

 

[BigWake]

Re: A little rattle never hurt no one...hopefully..

you sure the rattle is in the motor?

And yes 2strokes will do that as well.

 

[MadProps]

Re: A little rattle never hurt no one...hopefully..

absolutely it's in the motor, I swapped it to another boat for different reasons and the pinging/rattle is still there,

I guess I can check the timing and back it off some to test it

thanks for the reply on the pre detonation ping in 2 strokes

 

[lots of boats]

Re: A little rattle never hurt no one...hopefully..

Hi
If this was detonation it would probably be a dead motor by now. That high pitched two stroke ping you hear when a motor gets the wrong gas, too advanced timing, hot head etc. shuts things down. 120 compression good, brown plugs good, other places to look at are the coils, see if there is a gap in the high tension system some where and you hear a spark, look at your gear oil for metal, and decide if you are hearing gears.

 

[This_lil_fishy]

Re: A little rattle never hurt no one...hopefully..

Thanks, good point on the lower unit, gonna drain some fluid and give it a good check. I don't think it's pre-ignition as it only present at maintained full throttle and is a bit louder when I cut the throttle fast...then stops when the boat slows to match engine speed. Pre-ignition typically presents under load if I remember right.


Ian

 

[Art Burkett]

Re: A little rattle never hurt no one...hopefully..

Hi, I had a similar noise and it turn out to be cracks between the bolt holes holding the flywheel down. It made the motor sound like it was coming apart. Just a guess.

 

[This_lil_fishy]

Re: A little rattle never hurt no one...hopefully..

Definately not that, got it all tore apart now. Gotta do some sanding and painting on the hull today, will check the lower unit while the paint dries and I sit here looking out at a beautiful cloudless day.....