Loud knock when underload

[HBOlds455]

Long time reader first time poster. I really need your help... I have a 76 Avenger with a olds 455 and every time i put it in the water i get this knock that gets louder the faster i go. The bottom line is i just don't know where to start.

[Lumpy]

It could be many things. If it doesn't knock when you start it on the trailer but it does on the water my guess would be that you spun one or more rod bearings. Just a thought. Good Luck

[HBOlds455]

yeah it don't knock on the trailer only in the water.
Thanks for your reply

[djdtpr]

I would think that if it were a bearing in the motor it would make the noise on the trailer also at least every one i have ever heard have i would start with the pump or u joints dont overlook the simple stuff .Good luck let us know what u find.

[BowTie Rick]

Agree. I would start with drive line. The motor would knock on the trailer too if it were going south. Good luck, noises can be tough to locate origins on.

[Lumpy]

I had a jet once that spun 3 rod bearings. When I started it on the trailer it sounded fine but as soon as I put it in the water and a load was put on the jet it started knocking. You should be able to tell if it is coming from inside the motor or a u joint by using a stethoscope while it is running on the water (a funnel and a rubber hose works). Good luck.

[HBOlds455]

everyone thank you for your input....now i have an idea on where to start.

[propless]

HBOlds455
I honestly hope that it is something simple, I honestly do!
My own personal experience with 455's (too damn often - & now chevy), was spun rod bearings and rarely heard it if the boat was not under load.
I often compared the knocking sound to a little monkey inside the block with a hammer. If you get my point, I heard it often!
The 455 became a submerged reef in Lake Mead after I hit the block with an empty bottle of Jack Daniels, five freinds and I gave a prayer and kissed it good bye.

[rmclague]

This is only my expierience, but I would wonder
if you have over transom headers or through transom headers?
If you had a bad bearing it would let you know pretty quick!
They ussually do not run for long in that condition.
However open pipes will produce a detonation that sounds like a knock that will grow louder as you accelerate.
If the motor is running well other that that you may not be in as bad a shape as you thought.
Then again it may be cratered
rob

[djdtpr]

What does the oil pressure look like?If its a spun bearing you will notice some kind of drop in pressure u can also pull a valve cover that will tell the tale if there is metal shavings on top of the heads u will know whats wrong if not then u will know that it's not in the motor.

[Backtanner]

I had a similar problem on my BBC. It sounded great on the trailer and launch ramp but sounded horrible as soon as I stood on it. My problem was incorrectly set distributer points.

[Jetty 468]

HBOlds455:
Long time reader first time poster. I really need your help... I have a 76 Avenger with a olds 455 and every time i put it in the water i get this knock that gets louder the faster i go. The bottom line is i just don't know where to start.Your Topic and user name say's it all ! Sorry but been there tooooo many time's !! Running BBC now and it's the first time I made it through a season without spinning a bearing . Some have made it by driving the crap out of them but fact is you have a good chance of that with the olds !

[HBOlds455]

I want to thank everyone for there input. The loud knock was my drive line, fixed it on Saturday took it out on sunday and it runs great. Thanks again everyone

[djdtpr]

Cool deal its always the simple stuff that makes u scratch your head isnt it.