OMC 4.3L Rattle/Knocking Sound

AtomicMongoose

Recruit
Joined
Aug 16, 2017
Messages
3
Hello everyone,

My first post here and I need some help. I recently came into a 1988 Four Winns 200 Horizon for free. A family member had passed and the boat had been sitting for some time unused, however it is still in very good condition. Two weekends ago after a thorough going over and maintenance, we took it out for the first time. At first, all seemed well however after a few laps around the lake the engine lost power and RPMs and developed this strange knocking/rattling sound.

See video for an sample of the sound

https://youtu.be/WdKxWQYPkKw

Noticing a misfire as well, we did a compression test (all good there) and rebuilt the carb and ignition system. The engine starts instantly now, is very responsive, runs smooth and right up to proper RPM yet the rattle remains.

My father (an experienced auto mechanic) and I have been unable to diagnose what it could be. Several sources online report widely varying causes from rod bearings to gimble bearings and u-joins to engine alignment.

We are willing to fix whatever the issue is or have it done, but we really need to find out what it is first.

Thank you for taking the time to look and I'd appreciate any help in tracking down this noise. Really is a shame because the boat, otherwise, is great. Rides smoothly, shifts smoothly, ect so would quite like to get this fixed. Thanks again!
 

bruceb58

Supreme Mariner
Joined
Mar 5, 2006
Messages
30,454
Well I can tell you one thing, You burnt up your raw water pump impeller in the drive while running it without water. Run it for even 30 seconds and it is toast.

You can pull the drive and run the engine without the drive connected to rule out u-joints or gimbal bearing although I would think you would hear noise near the drive area. Always hard to tell in a video.

If you pull your drive and run it, make sure you supply water to the engine. Don't run it like that without water anymore.

If you ran it hard like that without water, your shutters in your exhaust may be falling apart and could cause this noise.
 

AtomicMongoose

Recruit
Joined
Aug 16, 2017
Messages
3
Ahh damn that quickly ehh? Well I goofed then. Well live and learn. I have an impeller kit and was planning on replacing it anyway though so I guess not a huge deal. Thank you though. I'll ensure I run it connected to water in the future.

Thanks for the suggestion. I plan on pulling the outdrive tomorrow and seeing. Appreciate the prompt response.
 

Scott Danforth

Grumpy Vintage Moderator still playing with boats
Staff member
Joined
Jul 23, 2011
Messages
47,306
Welcome aboard

In addition to your now melted impeller..... you will also want to check your flappers and your rubber exhaust hoses. without cooling water, the exhaust isnt only 200 degrees, its 1200 degrees and it burns the flappers and rubber hoses

never run a boat motor without water
 

Lou C

Supreme Mariner
Joined
Nov 10, 2002
Messages
11,802
The flappers could be vaporized and laying in the bottom of the exhaust housing, after you pull the drive you can look in with a bright flashlight and see if you can fish them out with a coat hanger wire. They are not hard to replace. The engine noise sounds bad. How is the oil pressure? I'd probably take off the exhaust manifolds and valve covers and see if you have a rocker arm bolt that is very loose, or a bent pushrod. That's about all you can really check without an engine tear down.
 

AtomicMongoose

Recruit
Joined
Aug 16, 2017
Messages
3
Flappers seem to be fine. Oil pressure is as well. Pulling the drive didn't stop the knock so not the gimble as some had suggested.

Now one rocker arm was quite loose but tightening didn't seem to fix it.

Assuming at this point it's probably a rod knock.

Looks like the engine is coming out...

Do appreciate the help everyone. Thank you
 

Lou C

Supreme Mariner
Joined
Nov 10, 2002
Messages
11,802
You might want to try adjusting all the lifters before pulling the engine. Not a hard job, you will have to remove the exhaust manifolds to get the valve covers off and mark the timing mark with paint on the harmonic balancer so you can get it lined up right.
 
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