Mercruiser 4.3 knocking noise

[Jrhinehimer]

I have a new to me boat I acquired over the winter. Engine is a mid to late balanced shaft non vortec 4.3. Went to start it this spring and the boat developed a knocking noise, one that I thought could be a rod/bearing. I pulled the engine and replaced it with a rebuilt long block. Now back in I still have the noise. Constant knock when started that gets faster with speed of the engine. I pulled the outdrive and started it and still have the noise. It’s hard to pinpoint the exact location but it seems to be the rear of the engine. I have narrowed it down to what I think may be the starter (although it starts fine) or the flywheel or coupler. Anyone have experience with something like this and have anymore recommendations? I will add that if I pull the coil wire and just turn the engine over without it starting I don’t hear the noise. This leads me to believe the starter may not be disengaging all the way but when I replaced the engine it looked visually fine and doesn’t appear to be that old.

 

[Scott Danforth]

welcome aboard.

how did you align the motor?

did you pull the drive then the motor, or just try to get the old motor out and the new motor in?

 

[Jrhinehimer]

welcome aboard.

how did you align the motor?

did you pull the drive then the motor, or just try to get the old motor out and the new motor in?

Pulled the drive and swapped motors and put the drive back in. Found I still had the knock so I pulled the drive back out. As it sits now with new engine and no drive I still have the noise. Would the alignment cause the noise even with the drive out? Side note, I have a new alignment tool and gimble bearing on the way that I had already ordered.

 

[Scott06]

Pulled the drive and swapped motors and put the drive back in. Found I still had the knock so I pulled the drive back out. As it sits now with new engine and no drive I still have the noise. Would the alignment cause the noise even with the drive out? Side note, I have a new alignment tool and gimble bearing on the way that I had already ordered.

No with drive off its an engine noise. Did you remove the drive belt off the engine to see if it is an issue with one of the front accessories?

 

[Jrhinehimer]

No with drive off its an engine noise. Did you remove the drive belt off the engine to see if it is an issue with one of the front accessories?

Yes, removed front belts and even front circulating pump.

 

[Scott06]

Yes, removed front belts and even front circulating pump.

only other longshot is if exhaust flappers in top of wye pipe are making the nocking… whats the history on the replacement engine?

 

[Jrhinehimer]

only other longshot is if exhaust flappers in top of wye pipe are making the nocking… whats the history on the replacement engine?

When I replaced the long block I replaced the exhaust manifolds and flappers.

 

[Jrhinehimer]

only other longshot is if exhaust flappers in top of wye pipe are making the nocking… whats the history on the replacement engine?

History on the replacement was I bought it from a local boat company that rebuilt it. Heads and all were gone through. All appeared well when I put it in. Used my intake and valve covers and accessories. holds good oil pressure and runs smooth.

 

[dubs283]

Did you check/adjust timing? Clean/rebuild/adjust the carburetor? Use the correct spark plugs?
If you can post a video with the noise, sometimes it's obvious to us, most times it isn't but worth a try

 

[Lou C]

It might be coming from an exhaust flapper I had this when I replaced the old OMC one piece V6 manifolds with the later 2 piece system, the water flow in them is different and the flappers make different noises than with the original exhaust. Darn near drove me crazy till I figured it out because I just did a top end overhaul at the same time! Used a mechanic's stethoscope to figure it out.

 

[Jrhinehimer]

Did you check/adjust timing? Clean/rebuild/adjust the carburetor? Use the correct spark plugs?
If you can post a video with the noise, sometimes it's obvious to us, most times it isn't but worth a try

Adjusted timing and new plugs wires and cap all ordered for this engine. Never touched the carb adjustment from one engine to the other. Runs/idles great. I’ll get a video

 

[Jrhinehimer]

Adjusted timing and new plugs wires and cap all ordered for this engine. Never touched the carb adjustment from one engine to the other. Runs/idles great. I’ll get a video

Adjusted timing and new plugs wires and cap all ordered for this engine. Never touched the carb adjustment from one engine to the other. Runs/idles great. I’ll get a video

 

[Rick Stephens]

Sure sounds to me like a bolt head or similar hitting the flywheel, or something like that..

 

[Jrhinehimer]

Sure sounds to me like a bolt head or similar hitting the flywheel, or something like that..

I thought that as well. I will add that the previous engine was a non balanced 4.3 and this one is balanced. I used the same flywheel cover and coupler. All seemed ok when I put this together but could there be a slight difference in the flywheel cover causing contact somewhere?

 

[Rick Stephens]

That part should be fine, no difference at the back end.

Only takes one long bolt though. Take a piece of broomstick, hold against your ear and touch different parts of the motor. El cheapo mechanics stethoscope.

 

[Jrhinehimer]

That part should be fine, no difference at the back end.

Only takes one long bolt though. Take a piece of broomstick, hold against your ear and touch different parts of the motor. El cheapo mechanics stethoscope.

I have a stethoscope and can’t seem to pinpoint the noise although it does get louder toward the rear of the engine.

 

[Jrhinehimer]

Any chance a coupler could cause this noise?

 

[Rick Stephens]

Any chance a coupler could cause this noise?

A coupler is a rubber filled steel cone with a plate and bolts holding it to flywheel. What's to make noise with nothing attached? I guess possible it is cracked through one of the ears, but even then, when nothing attached?

 

[Jrhinehimer]

That’s what I thought, just looking for ideas. Thanks

 

[Roadking57]

Definately sounds like a rotational eng noise. Its a short video, but my 2 cents is it sounds like a rod cap is loose, or a valve is way out of adjustment. You say its idling smooth so that rules out a valve adjustment.