what is this knocking noise? - 350 SBC - Team Camaro Tech
Troubleshooting Diagnosing problems done here.

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post #1 of 13 (permalink) Old Oct 28th, 18, 06:01 PM Thread Starter
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Tony
 
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Unhappy what is this knocking noise? - 350 SBC

My buddy has this SBC 350. The engine starts, idles and runs fine, but it has a nasty knocking sound under heavy load/acceleration. Compression was 110-115# in 6 or 8 cyl, (couldn't access the last 2 due to header placement). I need some ideas. Any questions or suggestions to help narrow this down would be greatly appreciated.

I got an error watching the video on this site, but it plays fine on youtube.
Copy and paste these links into your browser, (minus quotes).
"https://youtu.be/LSffr0O7SB4"
"https://youtu.be/Eq62Eexs_DI"



Last edited by Ghostbuster; Oct 28th, 18 at 06:44 PM.
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post #2 of 13 (permalink) Old Oct 28th, 18, 06:09 PM
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Don
 
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Re: what is this knocking noise? - 350 SBC

I’m gonna guess wiped cam lobe(s) but there are folks on here that are a lot smarter diagnosing this sort of thing than me.

Don

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post #3 of 13 (permalink) Old Oct 29th, 18, 04:08 AM
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Re: what is this knocking noise? - 350 SBC

when the cam went on mine it did not want to idle properly
easiest is to pull the valve covers and spin the engine and watch the valve, spring function
drain the oil in a clean container and examine it. cut open the oil filter. if there are particles that stick to a magnet, probably cam and or lifters. nonmetallic - bearings
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post #4 of 13 (permalink) Old Oct 29th, 18, 06:28 AM
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Re: what is this knocking noise? - 350 SBC

I had a cracked flex plate once that sounded like a knock under acceleration/load and idle to some degree. Many people listened to the sound and thought rod, wrist pin, cam ect.
I pulled up to a shop in San Diego and an old man in a rocking chair sitting out front told me to get my flexplate replaced as I stepped out of the car.
Tough to see the flex plate in the car so I'd start with the easy things....compression check on all 8, cut filter open and check for particles or maybe use a stethoscope to narrow down the origin of the sound.
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post #5 of 13 (permalink) Old Oct 29th, 18, 06:31 AM
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Re: what is this knocking noise? - 350 SBC

Guessing wiped lobe. Listen with a mechanics stethoscope at the valve covers and/or pull the valve cover and check the rockers
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post #6 of 13 (permalink) Old Oct 29th, 18, 07:27 AM
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Re: what is this knocking noise? - 350 SBC

Certainly sounds like valve train. He may simply have a bad lifter. Use a stethoscope as John suggested to try and isolate the cylinder. A wiped lobe is normally noticeable at idle.

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post #7 of 13 (permalink) Old Oct 31st, 18, 05:46 PM
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Re: what is this knocking noise? - 350 SBC

If it's the cam I'd bet on number 8.

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post #8 of 13 (permalink) Old Oct 31st, 18, 06:50 PM
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Re: what is this knocking noise? - 350 SBC

What is the “canister” to the left of the distributor in the second video that appears to really vibrate when it’s making the noise and stops when the noise seems to go away
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post #9 of 13 (permalink) Old Nov 1st, 18, 02:00 PM
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Re: what is this knocking noise? - 350 SBC

Looks like a possible add-on cruise control? My guess is also valve train related, even a possible bad valve spring, cam, lifter, or rocker stud, as others have suggested.
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post #10 of 13 (permalink) Old Nov 2nd, 18, 03:14 PM Thread Starter
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Re: what is this knocking noise? - 350 SBC

Very good George, that is correct, it's an after market cruise control.
We disconnected that item from throttle/vacuum and no change to the knocking under load sound.

I appreciate all the input from everyone, I was hoping it was something simple, it never is..

Last edited by Ghostbuster; Nov 3rd, 18 at 11:31 AM.
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post #11 of 13 (permalink) Old Nov 3rd, 18, 12:01 PM
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Re: what is this knocking noise? - 350 SBC

Quote:
Originally Posted by 68basecoupe View Post
What is the “canister” to the left of the distributor in the second video that appears to really vibrate when it’s making the noise and stops when the noise seems to go away
That's exactly what I think it is. The mystery canister vibrating and making contact with something.
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post #12 of 13 (permalink) Old Apr 27th, 19, 10:55 AM Thread Starter
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Re: what is this knocking noise? - 350 SBC

Turns out that cyl#3 was loosing compression, exhaust valve not seating completely. I'm told the noise under load, was detonation with the exhaust port open slightly. Time for a valve job on this 350.

Thanks to everyone who posted their thoughts on this problem.
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post #13 of 13 (permalink) Old Apr 28th, 19, 07:55 AM
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Re: what is this knocking noise? - 350 SBC

When I lost 3 lobes on my solid lifter cam, it made no noises what so ever, just popping through the carb at 5,000 rpm due to having both intake and exhaust lobes gone.

Mark 70 Z28 M22 3.73 Z21 Z87 - Lunati 20120121 Roller Cam

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