Knocking Mercruiser 140

[padn]

Hi Everyone

I have a 1979 Mercruiser 140. It has a dry knocking noise at idle (a little like a diesel) - it seems to continue at all RPM's but dissappers in the general noise. It seems to reduce when engine warm. It can hardly be heard with engine cover closed.

When RPM's are increased and hereafter reduced again the knocking dissapears for a few seconds to return again.

Using a timing light - the noise is syncronized with the flash.

The engine runs smooth and has plenty of power. Oil pressure is 40 at idle and 60-70 at WOT.

Crank and rods has been overhauled - main and rod bearings changed, piston rings changed, lifters changed and head changed.

Have adjusted the valves several times - though it is not at ticking noise.

Does anyone have an idea where to find the dry knocking ?

 

[dodgeramsst2003]

Re: Knocking Mercruiser 140

sounds like a piston slap noise from what you have described. When the engine was "gone through" was it bored over? was the cylinder taper and out of round measured? If everything esle was replaced and none of the above was done, piston slap could be the cause. Its usually worse when cold, and will go away when you take the load off. I had a 140 iron duke in my first boat. Always had a slight piston slap to it. Its not the end of the world, it will probably run for a long long time if thats what it is. As with any wear induced noise, it will get worse over time, but it will be a long time.

Chris

 

[fishrdan]

Re: Knocking Mercruiser 140

Yep, I'd bet it's piston slap. I did a re-ring on my Mercruiser 140 10 years ago and got piston slap afterwards, probably too aggressive with the ball hone... Honestly, my engine should have been bored with new piston's installed instead of just a re-ring job, I got what I paid for. While the noise was annoying, it didn't cause problems and the engine ran great for 10 years.

 

[Silvertip]

Re: Knocking Mercruiser 140

GM used to provide what were called "high limit pistons" and I suspect they still do. I have no idea if they are available after market or even through Merc but it's as simple as asking. If you get the "deer in the headlights look" you can assume they don't. These are pistons that were manufactured at the high end of the tolerance and are intended for rebuilds where things are just marginal or slightly outside spec. They don't always solve the piston slap problem but if you are on a strict budget and things are just a tad on the loose side, this may be an option.

 

[Coors]

Re: Knocking Mercruiser 140

A piston pin will make the same noise, as you describe.
Pulling the plug wire off will make it stop; not famialiar with piston slap.

 

[Bondo]

Re: Knocking Mercruiser 140

Pulling the plug wire off will make it stop;

Ayuh,.....

That's a pretty simple Test,.......Cheap Too.....