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Old 10-23-2012, 01:26 AM
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Chevy 350 engine knocking

My dad has a 1972 El Camino with 350 CI engine and 4B Carb. It has 56000 miles, and the car rarely gets driven. Recently, it has developed a loud banging noise in the engine similar to the sound of a backfire. It's a very clean engine, so I know its not the exhaust. Im not sure if it's backfiring through the carburetor. How am I able to see if it's backfiring through the carburetor, and if it is what is causing that problem? I thought it might be a sticking valve, however, I have done some research and found it could be a worn camshaft, bent rod, etc. My dad is weary about me working with his cars internally, so I would like to be able to pinpoint the problem before I start working. Before I truly begin I plan to replace the gas with fresh gas and change the oil. I hate seeing a sick beauty. PLEASE HELP!!

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Old 10-23-2012, 04:04 AM
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Its hard to pinpoint any problem relating to sound when there are so many different possible problems. If you can record the sound with your phone or camera and post the link on this site to youtube that would draw a better response.
Some possible abnormal noises from an engine can be rotating noises or reciprocating noises. A lot of noises sound similar from each of the following so you will need to ask an experienced ear at a garage if you are new to this.
1. Failed rod bearing
2. Bad lifter
3. Bad cam lobe
4.Cracked piston/ loose wrist pin.
5. Bent valve/ sticking valve in valve guide.
6. Cracked cast exhaust manifold
7. Blown head gasket
8 Old vee belts can develop a cracking sound.
9 Old bearings in water pump. grinding sound.
10 Loose camshaft thrust plate. ( cam moves back and forth and hammers lifters)
There are more than this which is why imo it is important to save yourself hours of chasing and get an experienced ear. Mechanics stethescopes are a valuable tool for pinpointing abnormal noises.
Goodluck
Al.
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Old 10-23-2012, 06:32 AM
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Dont rule out the ever present bad spark plug wire they could be causing it to missfire. Easy to test just by removing one at a time and replacing it until the sound changes. also will help find if the sound is coming from a cylinder fireing or from a rod knock. Rod will still make noise for the most part.

I have herd a flexplate bolt come loose and make an aweful sound. But it was easy fix.
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Old 10-23-2012, 07:16 AM
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Also could just be a bent pushrod or broken rocker arm. I would pull the valve covers and check those out first.
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Old 10-23-2012, 09:25 AM
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I've had a cracked flex plate make a low end rod knock type sound too, but it would go away once you gave it some r's which rods knocks usually don't do..lol
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Old 10-23-2012, 05:14 PM
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Quote:
Originally Posted by drainey48 View Post
My dad has a 1972 El Camino with 350 CI engine and 4B Carb. It has 56000 miles, and the car rarely gets driven. Recently, it has developed a loud banging noise in the engine similar to the sound of a backfire. It's a very clean engine, so I know its not the exhaust. Im not sure if it's backfiring through the carburetor. How am I able to see if it's backfiring through the carburetor, and if it is what is causing that problem? I thought it might be a sticking valve, however, I have done some research and found it could be a worn camshaft, bent rod, etc. My dad is weary about me working with his cars internally, so I would like to be able to pinpoint the problem before I start working. Before I truly begin I plan to replace the gas with fresh gas and change the oil. I hate seeing a sick beauty. PLEASE HELP!!
Do a compression test to see if there are one or more cylinders w/low compression. This could be caused by any number of things, but I'd be looking for a worn cam lobe, a broken valve spring or a bent pushrod for starters.

If a sparkplug has loosened or fallen out, that will make a loud popping noise, too.
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