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Old 09-14-2020, 04:20 PM   #1
Android04
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Knock after a simple carb swap - Thoughts?

Yes sir you read the title right... I've worked on TONS of SBC and BBC in my days and never have I had this issue. It may just be the luck of the draw in that exact moment. As I say "if I didn't have bad luck I wouldn't have any luck at all"...

Vehicle: 1985 Silverado Dually (C30)
Engine: 400 SBC
Trans: TH400

So I've been working through some leaks the truck has had and got it running and switching gears. New intake manifold gasket, new valve cover gaskets, new thermostat, and all new fluids. Then noticed the governor was chewed up so I put in a new one in the trans, with a new trans filter, gasket, and fluid. Yesterday I decided to drive it a short distance and it did great. I knew the carb was having some jet issues.

So I swapped out the carb for once I had laying around that was perfect on a previous project that is now on Holley. Swapped it on my truck, started it, idle was a little high but seemed like the throttle cable was too tight. Adjusted it and BOOM... A super loud knocking sound. Something it has NEVER done.

I first thought it was a fly wheel bolt maybe, or something smacking. No luck. So I pulled all the plug wires off one by one and the noise or pitch never changed. Still has great oil pressure. But if I start it and turn it off it sounds like a rocker arm is still moving for a solid 30seconds, and sounds like it back by #8.

Although I'm afriad its in the bottom of the motor I wanted to see if anyone had any suggestions, thoughts... etc.
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Old 09-14-2020, 05:40 PM   #2
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Re: Knock after a simple carb swap - Thoughts?

Pull all the accessory belts off and check if the noise is still there. A bad PS pump can sound like self destructing engine guts.

Might check the 'new' carb for missing screws, nuts and parts. Maybe something was loose and is now rattling around in a cylinder. Since the noise seems to come from #8, pull the plugs for both 6 & 8 and check for damage or other weirdness.
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Old 09-14-2020, 06:02 PM   #3
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Re: Knock after a simple carb swap - Thoughts?

Go grab the carb you took off and look underneath it for the bolts that hold the base plate to the main body. Good chance one fell out and went down the intake into a cylinder when you pulled it...ask me how I know.
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Old 09-15-2020, 08:36 AM   #4
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Re: Knock after a simple carb swap - Thoughts?

Sometimes the fuel pump can make a solid knocking sound all of the sudden. Try the old broom handle-to-the-ear trick.
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Old 09-15-2020, 09:06 AM   #5
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Re: Knock after a simple carb swap - Thoughts?

Quote:
Originally Posted by Dead Parrot View Post
Pull all the accessory belts off and check if the noise is still there. A bad PS pump can sound like self destructing engine guts.

Pulled the belts off, and still the same noise. Thanks for the suggestion

Might check the 'new' carb for missing screws, nuts and parts. Maybe something was loose and is now rattling around in a cylinder. Since the noise seems to come from #8, pull the plugs for both 6 & 8 and check for damage or other weirdness.
I checked the old carb and nothing was missing, going to pull the new one off today and check it if I can get some time. Also going to pull the plugs and just double check all is well. I will post back with my findings. If nothing after this I'm guessing a compression check.

Quote:
Originally Posted by Corts60 View Post
Go grab the carb you took off and look underneath it for the bolts that hold the base plate to the main body. Good chance one fell out and went down the intake into a cylinder when you pulled it...ask me how I know.
Quote:
Originally Posted by kidatheart View Post
Sometimes the fuel pump can make a solid knocking sound all of the sudden. Try the old broom handle-to-the-ear trick.
I forgot about that old trick, I will do that today as well.

I'll post back with what I find if anything at all. Thank's everyone.
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Old 09-15-2020, 09:07 AM   #6
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Re: Knock after a simple carb swap - Thoughts?

Quote:
Originally Posted by kidatheart View Post
Sometimes the fuel pump can make a solid knocking sound all of the sudden. Try the old broom handle-to-the-ear trick.
That didn't work so well for Jeffery Dahmer.
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Old 09-15-2020, 09:09 AM   #7
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Re: Knock after a simple carb swap - Thoughts?

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That didn't work so well for Jeffery Dahmer.
Haha, I watched his documentary the other night.
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Old 09-17-2020, 10:11 PM   #8
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Re: Knock after a simple carb swap - Thoughts?

You could have a piece of carbon lodged on top of a piston. Will knock like crazy. Should be able to feel it if you turn the engine by hand . There are engine cleaners for this. I think GM use to have a Top Engine cleaner to run through one. I watched an older mechanic fix a 4.0 liter jeep with a can of water that was knocking. I've also heard of running trans fluid through the carb to burn the carbon off. Hope its something on a piston rather than a bearing.
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Old 09-18-2020, 07:08 AM   #9
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Re: Knock after a simple carb swap - Thoughts?

Good luck with your diagnosis.
Here is a tool I used for years to pin point noises.
The link below is just an example,you can get them alot of places.
Link: https://www.northerntool.com/shop/to...d4d0d69d744892









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Old 09-18-2020, 07:34 AM   #10
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Re: Knock after a simple carb swap - Thoughts?

A boroscope is a great tool to have for looking inside the cylinders. I suggest that if you get one, make sure that it has a screen that works on its own - no wifi needed. About $60 for a decent one. Perhaps a washer fell into the intake. It can make a heck of a noise. Check plugs to see if any gaps are mashed or damaged.
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Old 09-22-2020, 04:06 PM   #11
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Re: Knock after a simple carb swap - Thoughts?

Thanks everyone for replying.

So I changed all fluids, checked all plugs (no issues), and took off the aux belts. Started it up before I left for the Rod Run, and the noise went away but would randomly come back for a split second and then gone again.

I left it sit while I was out of town, and came back last night. I checked all fluids today, and then started the engine. NO NOISE AT ALL! Rev-ed it up and let it sit running for about 5min. No smoke, no noise, no nothing. So I shut it down, and got under it and checked the bolts for the flywheel to the torque converter. None of them were loose but I noticed that when I turned it, it had a slight wobble or out of center. I gave a little strong arm action and then re-started the truck.

I got back under it and the torque converter and flywheel are definitely not running straight and I could hear a slight tapping noise from the bottom of the trans pan. I was afraid it was the motor but seem's to be the trans. Don't know anything about the auto-trans other then what I've learned with this one, so we shall see.

Still going to do a compression check and run a camera through the cylinders to make sure. Thanks again for all the replies and suggestions.
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Old 09-22-2020, 08:10 PM   #12
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Re: Knock after a simple carb swap - Thoughts?

With that info you just gave.
I would say your Flywheel is crack around the bolt circle.
I have had that happen before.
It is a (Clicking /Knocking sound)

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I was at the Rod Run.
It was a Big Crowd







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Old 09-22-2020, 08:14 PM   #13
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Re: Knock after a simple carb swap - Thoughts?

Quote:
Originally Posted by Getter-Done View Post
With that info you just gave.
I would say your Flywheel is crack around the bolt circle.
I have had that happen before.
It is a (Clicking /Knocking sound).
I was at the Rod Run.
It was a Big Crowd
.
Yeah unfortunately that's what I'm thinking as well.

Yes sir it was, but great weather and a great show other than all the "Cali-lean" Teenagers trucks driving around. I'll never understand that trend.
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