Loud thumping/knocking at low rpm, '86 4.3

Joined
Sep 3, 2017
Messages
12
Hi all,

I'll hang around and help others to the best of my ability if you can clue me in on what might be wrong with my engine/drive.

Boat in canada at Dad's, me in US. Put new crate engine from Summit racing in back in 2019, broke it in properly, only about 50 hours on engine. Then covid hits, boat is outside for 2 years while I can't get to Canada, Dad is 89 years old and health issues, can't ask him to help. Covered engine etc as best we could but still got corrosion - we live next to ocean in BC.

Tried to start last year, got terrible "PANG!" sound under valve cover. Intake valve on #3 cyl rust welded shut. Rocker arm post got knocked out pushrod bent, etc. Unable to get up after that due to work, sat until last month. Pulled engine, figured out about valve, Tapped out post and added threaded post. Fixed valve, new head gasket, etc. All back together, properly(I think) pre-load rocker arms/pushrods, everything back in boat.

Start it up, purrs like a kitten above ~1100 rpm, nasty thumping sound in engine (I think) below 1100, almost like a missing rod bearing or something.

My next move is to pull the outdrive and start it up and see if sound is still there.

Any ideas on what it could be greatly appreciated before I go ahead and pull the engine out again.

Thank you,
Rich
 

alldodge

Supreme Mariner
Joined
Mar 8, 2009
Messages
36,580
Intake valve on #3 cyl rust welded shut

Rust on valve, means water got into the motor and may have also rusted a rod or main bearing. Intake gasket would be first thought. When old gasket was removed did you see anything indicating a leak?
 
Joined
Sep 3, 2017
Messages
12
Rust on valve, means water got into the motor and may have also rusted a rod or main bearing. Intake gasket would be first thought. When old gasket was removed did you see anything indicating a leak?
No no leak on intake I could see, and used a new gasket when I put it back together. This is a very jarring, metallic sounding knock/thump. Water definitely got into the engine so I'm thinking you may be right about the lower end.

Planning to pull engine back out today and have a look.
 
Joined
Sep 3, 2017
Messages
12
Ok, so pulled engine out put on stand, flipped over and removed oil pan. Everything looked very good. There was a little bit of surface rust on one of the crankshaft counterweights, so little it basically rubbed off with a paper towel.

Nothing appeared wrong with the bearings etc. Nice and tight like you would expect from a new engine.

What I DID find after I pulled the engine out was that apparently I hadn't fully tightened the studs/nuts that hold the flywheel and that big rubber doughnut thingy that the driveshaft from the leg goes into. Without them being torqued down the flywheel has about 1/4" clockwise/counterclockwise play in it. Thinking that may have been my thumping sound.

Putting it all back together tomorrow to test.
 

alldodge

Supreme Mariner
Joined
Mar 8, 2009
Messages
36,580
Flywheel loose, that should make some noise. The rubber thing is the engine coupler

Use blue loctite on bolts
 
Joined
Sep 3, 2017
Messages
12
Ya, just to wrap up the thread, it looks like it was just the flywheel. Once I got it tightened down the noise went away.

Thanks everyone for the responses!
 
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