knockin 4.3 mercury

brad24

Cadet
Joined
Jun 26, 2013
Messages
6
My engine started knocking sound like a rod but it goes away and I only hear it when am at full throttle and trimed up after I trim down and let off a little it slowly goes away. It was begining of the season was thinkin maybe bad gas I ran it out nowbut kind of sscared to fill it up and try it again any ideas
 

OldNBold51

Petty Officer 2nd Class
Joined
Apr 19, 2010
Messages
169
Re: knockin 4.3 mercury

it may be the u joints in the outdrive knocking when you have it trimmed up but disappears when you trim down.
 

brad24

Cadet
Joined
Jun 26, 2013
Messages
6
Re: knockin 4.3 mercury

it may be the u joints in the outdrive knocking when you have it trimmed up but disappears when you trim down.
I have to back off the throttle for it to stop knocking and it takes i couple mins to stop aftet i back off but it starts as soon as I trim up
 

bonzoscott

Senior Chief Petty Officer
Joined
Jul 26, 2008
Messages
745
Re: knockin 4.3 mercury

Try full throttle without trimming up. If you still hear it, probably a rod.
 

Watermann

Starmada Splash of the Year 2014
Joined
Jan 12, 2013
Messages
13,747
Re: knockin 4.3 mercury

Trimming the outdrive up or down won't make or stop a rod from knocking. Make a video of the motor running so guys can hear what the sound is your concerned about.
 

Volphin

Lieutenant
Joined
Jun 5, 2011
Messages
1,405
Re: knockin 4.3 mercury

Uh, no. Bad gas will not sound like a rod knock.
 

500dollar744ti

Senior Chief Petty Officer
Joined
Jul 23, 2012
Messages
691
Re: knockin 4.3 mercury

Not always, rod knocks can come and go with changes in engine load and oil pressure, I've seen a lot of weird stuff.

If the rod is knocking at idle, disconnecting the ignition lead to the cylinder that's knocking will cause it to quiet down or completely go away, then you know it's a knock for sure.

What's your oil pressure like, that can tell you a lot.
 

old islander

Petty Officer 1st Class
Joined
Jan 27, 2013
Messages
302
Re: knockin 4.3 mercury

My engine started knocking sound like a rod but it goes away and I only hear it when am at full throttle and trimed up after I trim down and let off a little it slowly goes away. It was begining of the season was thinkin maybe bad gas I ran it out nowbut kind of sscared to fill it up and try it again any ideas

I'm betting on a u joint....but I lose every time I go to a casino.
 

Volphin

Lieutenant
Joined
Jun 5, 2011
Messages
1,405
Re: knockin 4.3 mercury

Ayuh,.... But, Bad gas causes Detonation rattle, which leads to a rod knockin',....

It sure does! Detonation will kill an engine quicker than you can say gasohol. LOL
 

brad24

Cadet
Joined
Jun 26, 2013
Messages
6
Re: knockin 4.3 mercury

Ran the boat this weekend put new gas in it and fuel treatment it did not knock at all even at full throttle trimed up so must have been the gas
 
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