Knocking in motor
Thread Starter
Drifting
Joined: Apr 2002
Posts: 1,716
Likes: 4
From: New Baden Illinois
Knocking in motor
On Monday, I put my quadrajet back on my 76 L48. I had just gotten done rebuilding it. Naturally, I had to take it for a test drive. After getting done with my test drive (only took about 7 or 8 minutes and doing about 120 mph), I turned on to my street (about 2 minutes after running at high speeds). All of a sudden, I got a really loud knocking coming from the motor. The knocking got faster if I gave it any gas, so I idled down the street and pulled into my driveway. When I opened the hood and started it back up, it was still there and pretty loud. Tonight, I took the valve covers off to check and see if I may have broken a rocker, spring, or pushrod. None of the above. I am not sure what inside the motor could be making that knocking noise. Could a crank bearing actually have come out and the connecting rod is riding directly on a crank journal. Is is something simple (yeah right!!) or is the motor going to have to come out. Anyone with ideas, please help. Thanks.
Thread Starter
Drifting
Joined: Apr 2002
Posts: 1,716
Likes: 4
From: New Baden Illinois
Re: Knocking in motor (Jvette73)
I do have an automatic. I have never heard of that. Is there an easy way to check that, or an easy way to check and see if it IS that? How does it mimmick that noise? What is actually knocking against each other?
Safety Car
Joined: Jun 2001
Posts: 4,004
Likes: 2
From: Severna Park Md
Re: Knocking in motor (Jvette73)
I suspect you were having carb troubles and thats why you rebuilt it? If your carb was loading up and flooding raw fuel into the intake, it can run down past the rings and into the oil pan and thin out the oil. If you catch it right away and fix the carb and change the oil all should be fine. Run it like this for too long and the engine wont last long.
The flexplate will knock back and forth against the convertor and bolts and such. There are 3 that hold the flexplate to the convertor. You can get to them by removing the convertor inspection cover. You'll have to rotate the engine to get to them all. This knock sounds like a rod knock but its not quite as consistant. You tend to hear it more when you bring the rpms up and down a little. A serious rod knock will be consistantly knocking even at idle.
Hows the oil pressure?
The flexplate will knock back and forth against the convertor and bolts and such. There are 3 that hold the flexplate to the convertor. You can get to them by removing the convertor inspection cover. You'll have to rotate the engine to get to them all. This knock sounds like a rod knock but its not quite as consistant. You tend to hear it more when you bring the rpms up and down a little. A serious rod knock will be consistantly knocking even at idle.
Hows the oil pressure?
Thread Starter
Drifting
Joined: Apr 2002
Posts: 1,716
Likes: 4
From: New Baden Illinois
Re: Knocking in motor (Jvette73)
Actually, I had to rebuild the carb because the throttle shaft bushings were leaking vacuum. I had those professionally replaced and I did the rest myself.
Also, the funny thing was, I glanced at the oil pressure guage right when I started hearing it. It was normal (right under 40 p.s.i since I was idling).
But, one thing, it does knock at idle, it just speeds up when the rpm's go up. When I had the valve covers off tonight, I slowly turned the motor over to see if all of the valves on the left side were moving ok and they were. I didn't hear any noise then. It wasn't until I actually got it to fire up again that I heard the noise. :confused:
Also, the funny thing was, I glanced at the oil pressure guage right when I started hearing it. It was normal (right under 40 p.s.i since I was idling).
But, one thing, it does knock at idle, it just speeds up when the rpm's go up. When I had the valve covers off tonight, I slowly turned the motor over to see if all of the valves on the left side were moving ok and they were. I didn't hear any noise then. It wasn't until I actually got it to fire up again that I heard the noise. :confused:
Racer
Joined: Oct 2001
Posts: 274
Likes: 2
From: Pampa Tx
Re: Knocking in motor (dgruenke)
I hate to ask this, but if you had the carb butterflies out, did you ping or locktite the screws so they couldn't come out that retain the butterflies to the shaft. I have seen a couple of engines where a screw dropped into the engine. One from a butterfly, the other was the bevelled screw from inside the air horn. The first just roughed up the piston and didn't hurt the head, but the second one cracked a head.
Phlash 72 BB Roadster :rolleyes:
Phlash 72 BB Roadster :rolleyes:
Thread Starter
Drifting
Joined: Apr 2002
Posts: 1,716
Likes: 4
From: New Baden Illinois
Re: Knocking in motor (Phlash)
I am not sure what was used for the butterflies. The shop that I took it to specialized in carb and fuel system repairs. They replace bushings on these things about once a day. Which screw is the other screw that you were talking about. Is it one of the air horn hold down screws?
Racer
Joined: Oct 2001
Posts: 274
Likes: 2
From: Pampa Tx
Re: Knocking in motor (dgruenke)
The butterfly screws can come out easily if they didn't ping them or use locktite. The larger screws that can come out if not properly tightened are just inside the air horn under the choke. Also the two by the air breather stud can do the same. Any of these can cause significant damage. If you are missing a screw on the carb, pull the plugs and there is a good chance the affected cylinder can be found by the beat up electrode. Good luck
:seeya
Phlash 72 BB Roadster
:seeya
Phlash 72 BB Roadster
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Racer
Joined: Dec 1999
Posts: 351
Likes: 0
From: Connecticut
Re: Knocking in motor (dgruenke)
You can also unplug one wire at a time while the engine is running. Use sense when doing so, If the sound goes away on one its a spun bearing. Lets hope not.
[Modified by chevyman, 9:43 PM 4/5/2002]
[Modified by chevyman, 9:43 PM 4/5/2002]
Safety Car
Joined: Jun 2001
Posts: 4,004
Likes: 2
From: Severna Park Md
Re: Knocking in motor (dgruenke)
Try this: Connect the clip end of a test light to ground and start the engine. Put the point of the test light between the boot and plugwire insulation near the cap to ground out the cylinders one at a time. By killing the spark to the cylinders you may be able to hear a change in the noise on one of them. It could indicate the actual cylinder with the bad rod bearing. Not that it will fix it but it may help in diagnosing. If its really bad, I suppose it could knock even with a dead cylinder. Just an idea.
Thread Starter
Drifting
Joined: Apr 2002
Posts: 1,716
Likes: 4
From: New Baden Illinois
Re: Knocking in motor (Jvette73)
I am a little confused. How will preventing a spark plug from firing stop a noise that is caused by a spun bearing. Doesn't the noise just occur from the rotating assemby turning. If it does, it would still make the noise. Is it because there is less force on the bearing? :confused:
Racer
Joined: Oct 2001
Posts: 274
Likes: 2
From: Pampa Tx
Re: Knocking in motor (dgruenke)
If the spark plug is functioning, then the power from the detonation drives the rod against the crank. If there is a bad bearing, typically there is too much clearance, which the knock comes from the rod striking the crank. By eliminating the explosion or detonation in the cylinder chamber, no power is transfered to the crank, thus lessening the knocking sound. There may still be some knocking as the crank pushes against the rod on the compression stroke and on the exhaust stroke, but the sound should be less in intensity.
Good luck :seeya
Phlash 72 BB roadster
Good luck :seeya
Phlash 72 BB roadster
Thread Starter
Drifting
Joined: Apr 2002
Posts: 1,716
Likes: 4
From: New Baden Illinois
Re: Knocking in motor (dgruenke)
Alright, I found the problem but I am still thinking about how to fix it. Phlash, you were right. A carb screw dropped down. It wasn't a butterfly screw or an air horn screw. It was the tiny screw that holds the choke rod to the choke plate (the one that controls the air opening on the top of the carb). I have never heard of anything like that happening before.
By the way, the way I found the problem was because I was starting the engine with a spark plug wire off at a time and I kept getting backfire up through the carb. Finally, I got a flame burst and the I knew something was up with the carb. When I noticed that the choke was wide open, I also noticed the missing screw.
Next question, does anyone know of an easy trick to get it out of the hole without taking the head off? I figured I would just pull the plugs to find out which hole it went down. Also, how can I make sure that it didn't damage any of the valves?
Thanks to everyone for all of their help so far. :D
By the way, the way I found the problem was because I was starting the engine with a spark plug wire off at a time and I kept getting backfire up through the carb. Finally, I got a flame burst and the I knew something was up with the carb. When I noticed that the choke was wide open, I also noticed the missing screw.
Next question, does anyone know of an easy trick to get it out of the hole without taking the head off? I figured I would just pull the plugs to find out which hole it went down. Also, how can I make sure that it didn't damage any of the valves?
Thanks to everyone for all of their help so far. :D
Racer
Joined: Oct 2001
Posts: 274
Likes: 2
From: Pampa Tx
Re: Knocking in motor (dgruenke)
The best thing to do would be to pull the head, if you can determine which side it is on. Otherwise you won't know the extent of the damage. If it just roughed up the center of the piston you might be ok, but if it cracked the outer edge of the piston, you are looking at a potential major failure if you don't do the repairs now. There is also the possibility that it already messed up the seat, valve, and even cracked the head. If it were mine I would at least start with pulling it down to the block. :U
Sorry for the news!
Phlash 72 BB Roadster
Sorry for the news!
Phlash 72 BB Roadster
Thread Starter
Drifting
Joined: Apr 2002
Posts: 1,716
Likes: 4
From: New Baden Illinois
Re: Knocking in motor (Phlash)
That is what I was afraid of. I pulled all of the plugs and none had damage, so I have no idea which hole it is even in. It is a pretty loud knock though. I am just afraid to pull the heads off and it end up being a spun bearing and have to put it back together to pull the motor out. Although, I think it would be too coincidental for that screw to fall in there and then a bearing come loose. I guess the only thing left is to tear it apart. Thanks for your help. :cheers:
Drifting
Joined: Feb 2000
Posts: 1,607
Likes: 0
From: Clarksville,Tn,USA TN
Re: Knocking in motor (dgruenke)
Hey, I know how you feel. The first motor I built for the vette developed a knock I could not adjust out at the rockers so I tore it back down. It sounded the loudest coming from the intake area on the driverside. Anyway, when I pulled the heads I had some marks on top of a couple of pistons. In the end, a couple of valves got bent and I had to replace them. I have been doing a frame off on my 78 for a while and the motor has been done. I figure my little helper 3 year old may of dropped something down a plug hole or 2. The other option was, perhaps something was in the carb and fell in when I rapped on it. It was in a shoebox with a bunch of other stuff in it. Oh well, live and learn for me. I pulled the valves and checked them and only 2 were bent. Could have a bent valve making a knock.
Neil in Tenn
Neil in Tenn
