Engine knock after carb change

morgan in ar · Aug 11, 2019

I just put a rebuilt carb on my 65 Chevy c20 with a 283. The engine sounded good before, but it would sometimes flood out at idle. Immediately after I swapped the carb, it is knocking. And I mean loud. I can't really tell where it's coming from. I'm afraid there was something inside that carb that the engine ingested. Any other ideas?

grizz02 · Aug 11, 2019

that would be my guess , something got drop down the intake , pull all the plugs look at them to see if there are any showing damage , a compression check might tell which cylinder it is, if has bent a valve

warbaby · Aug 11, 2019

knocking all the time, or just under a load?

Steve@Advance · Aug 11, 2019

Were all the screws accounted for in the bottom of the carb?

One may have fallen out when it was removed.

Not good...

morgan in ar · Aug 11, 2019

All the time

morgan in ar · Aug 11, 2019

I didn't lose any hardware, I am afraid something fell out of the old or new carbs. I've only ran it long enough to see if it seemed fuel or spark related. I'm absolutely sick about it. It ran pretty good, carrying great oil pressure.

john in la · Aug 11, 2019

What Steve is trying to say is look at the bottom of the old carb and see if a screw is missing.
Sometimes a screw will be broken and when you pick up the carb to take it off the screw will fall out.

You can also check if a pin is missing from the linkage that fell off while you were removing carb.

But 6 one way half a dozen the other.
If it is knocking the damage is done.
So sorry.

2x4 · Aug 11, 2019

had the nut fall off the air filter lid & go down & hit the piston. Dented the top of the piston but didn't really hurt it. After several hours the knock went away. I wouldn't give up on it yet. 261 6 cyl. in 1960 C60.

shinnery · Aug 12, 2019

I rebuilt the engine from a '56 Chrysler Windsor and apparently had something in the air cleaner. Put it on and engine started missing bad. Whatever it was got all four cylinders off one side of carb. Had to do a complete valve job. That has to have been over 50 years ago.

Nat · Aug 12, 2019

Have you checked to see if the oil is overfull? I had an old Dodge do that and it had leaked a lot of gas into the oil pan and made the oil thin as gasoline and it knocked. I dfrained/replaced oil and filter and it was fine then

Geo-TH,In · Aug 12, 2019

HF sells a camera with a 3 ft flex cable to see inside cylinder.

I would pull all plugs and take a look see inside.

If you were in Terre Haute, I would lend you mine.

ASEguy · Aug 12, 2019

Geo's idea is a good way to start. I've seen small things get sucked out of one cylinder and into another, so I would not run it anymore until I verified the issue.

rustred · Aug 12, 2019

I have never seen that happen.

rustred · Aug 12, 2019

but years ago have seen many engine knocks from carbon on the pistons. would swear its a rod brg.

Steve@Advance · Aug 12, 2019

John, remember the old 4 barrel Rochesters that had a 3/8" x 1 1/2 slot head screw in the base of the carb?

I pulled an intake off once, found that screw wedged crossways inside one of the intake ports in the head!

What luck that it wedged sideways instead of getting on top of the valve! Probably been there a long time.

Steve@Advance · Aug 12, 2019

If you can verify which cylinder it's in with the borescope, might can just pull that head, see what happened.

If the piston is not dinged too bad it will be OK. But if the outer edge is hit, chances are the compression ring is trapped and not floating in the groove as it should.

With the head off, you can fill the ports with diesel or similar, watch for leakage in case it bent a valve.

Good news, if any, it's about as easy to work on as you will find.

dzc3 · Aug 12, 2019

May not work but worth a try pull all the spark plugs and whirl the engine over if its loose it may blow out. If it still knocks, do what others suggest.

David G · Aug 12, 2019

Sounds like you dropped something down the intake.

Mutt and Jeff · Aug 12, 2019

If automatic it could have broken the flex plate (flywheel thingy)
or maybe blown an exhaust gasket or even now firing on that cylinder that had one.

Get out a 4 foot chunk of garden hose
hold one end to your ear and track down the sound with the other

Determined · Aug 12, 2019

If you think something has gone into a cylinder;

Pull plugs and look for a damaged one or one with no gap on it.

Some small things can be fished out through the plug hole.

If not the next step would be pull valve covers and check for a bent valve or pushrod, if something got stuck and held a valve open the piston may have smacked it.

If no luck with the above then you will probably be pulling a cylinder head next.

JDBRIDER · Aug 12, 2019

Do you think you may have gotten some small debris into the manifold? In my High School days I had a 56 Chevy with a 265 V8. I took the carb off, don't remember why. Anyway on those old cars there was an exhaust passage up thru the intake manifold to the bottom of the carb to prevent carb icing. When I took the carb off 2 of the mounting bolts twisted off. They were burnt nearly in two because of the exhaust ranning thru there. I carefully drilled out and retapped the bolt holes. Everything went OK til I started the car. It knocked something awful. I quickly shut it off. I couldn't imagine what had happened. I went to town to my local service station and asked them what it could be. They said that I had probably let a few drillings fall into the manifold and they got sucked into the cylinders. There they got hot and were glowing causing pre ignition. I couldn't imagine that could cause such a loud knock.
They said to just start the car and let it idle til the drillings burnt away. I let it idle and knock for a few minutes then all was fine.

Engine knock after carb change

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