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knocking noise 1984 140

E

eddiefitz

New member
I have a 1984 century with a 140hp 3liter with an alpha one stern drive. Had it running in the driveway with muffs on today and everything was fine. Got it to the launch, fired it up and this loud knocking noise was coming from (I think) the block area. It was still in neutral when this was happening. Let it warm up and the noise went away. Got underway and the boat seemed to be cavitating at slow speed, I gave it a little juice and the knocking came back and the engine wanted to die. I limped it back to the dock, got it home, put the muffs back on, and the knocking was gone. Engine oil seems fine, I pulled the top cover off of the stern drive, and that oil and bearing seem fine. No other symptoms were present. I cant figure it out. My guess would be a u-joint in the lower unit, but if that were the case, why would it knock in neutral. Thanks in advance for any help or suggestions.

Andy T.
 
Re: knocking noise

there ain't no u joint in the lower, it's in the rubber boot between the drive and the transom.

How high up was the drive trimmed when you were running it.
 
Re: knocking noise

Not susposed to run engine/outdrive with the outdrive all the way up.

Puts a lot of strain on the ujoints and gimbal bearing.

Always trim outdrive down ~ 1/3 to 1/2 way down before starting....

Also you may have multiple issues and you may think the noise has something to do with the engine running.

Did you service the ujoints and gimbal bearing???

They must be serviced every season and or 50 - 100 hrs.
 
Re: knocking noise

Do not run the engine with the drive up, this will cause knocking and wear the u-joints. Hopefully you have not caused any damage. If, while running at low speed, you hear knocking then the problem needs to be found and addressed. Knocking sounds generally start to be noticed when going slow and turning hard over either direction. U-joints and gimbal bearing can cause knock/noise.
 
Re: knocking noise

I guessed right ?

Not bad for the first time. HAH !

Yea you should really pull the drive and have a look at the u-joint and bearing.

Need a free online manual, i'm sure if you ask nice someone here will give you the link.
 
Re: knocking noise

ok, pulled the drive, ujoints and gimbal bearing look to be ok, will take to marina to have inspected just to be on the safe side. I decided to pull the spark plugs to see if there was any indications up there, and sure enough #3's electrode was bent over, not smashed but definetely bent.
My guess is that when I recently rebuilt the carb, I accidently dropped something down the intake(dang it!!) Not for sure that this was the cause of the knocking noise, but I think I found where the knocking was coming from. I did a compression check, but after words noticed that you have to have the throttle plate open, so I will do it again and hope the results are different
#1=125, #2=80, #3=80, #4=150
 
Re: knocking noise

ain't going to make a different with the plate open your gonna get less compression.

Head got to come off. Put it on the bench. Then look at the pushrods. They bent at all? Roll em on a flat surface.

I use gas you use what you want and pour the liquid on all the valves they leaking you will know cause it will disappear. Valves good and seated good ? Yes ? Put aside the head. That ain't your problem then.

Now it's time to look at the pistons. Get a magnifying glass, and really look at the pistons and walls.

You find the nut washer or whatever you dropped in there.

My friend dropped a washer in also exact same thing as you we found it jumped around between 2 cylinders also, found pieces on top of both middle cylinders.

No damage new head gasket and it was good to go.

Ok advice from a old geezer me, to you. Up to you to accept it or blow it off.

Don't beat yourself up over this. A mistake was made, that only proves your human.

Bet you learned from it. So make up your mind your gonna fix it, and forget it.

Need more help post and we all here will help ! And i bet there is not one guy here that hasn't made a mistake.
 
Re: knocking noise

That's great advice, and I really appreciate all of it!! I am still wondering if the rings are worn. I put a tsp of oil in both low cylinders, and it brought them both up to 90 psi. Someone else I talked to said carbon could have caused the spark plug issue. Not sure I want to stick the money into the boat on a complete rebuild, I think I can handle the head gasket. Does anyone have any links to a step by step guide on replacing the head gasket.
Again, I really appreciate all of your help!!
 
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