1981 Chev Caprice - 305V8 Flat Cam

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92zman
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1981 Chev Caprice - 305V8 Flat Cam

Post by 92zman »

We have a 1981 chevy 305 that is making a weriod noise coming from the carb and it falls on it face and i have the boss yelling at me then it is a flat cam. he says then they all do it. I have never seen a cam in a 305 go bad but that is me. Any one see them go flat?
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steven kiser
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Re: 1981 Chev Caprice - 305V* Flat Cam

Post by steven kiser »

Well going way back in the time machine I've replaced a few, or more than a few. I believe G.M. had a recall on these that was good for a 100 k. May have been earlier, can't really nail it down. If one of my techs was working on it with the conditions you describe I would instruct him to put the vehicle in the air and disconnect the knock sensor. I believe it's near the starter motor on this vehicle. If this eliminates the flattening it's doing it's job. It's picking up the knock as a detonation knock and is retarding the timing. There was a crossfire issue with these that could be causing the weird noise by the carb. They would detonate inside the exhaust manifold and you would hear it through the carb. See if there is a hot spot on the manifold, it will stand out against the rust. It usually looked like someone sprayed a spot with silver / blue paint. Another issue these had that was horrific was the cats backing up. Just like someone shoved a tater up the exhaust pipe. If you start it give it gas and it sounds like a jet trying to take off or whistles then flattens right out they're probably plugged.

When we had them in the shop for cats being plugged (when they were out of the extended warranty) and I think yours is if it's over 100k, we would always tune them as well. Plugs would overheat causing a resistance issue leading to cross fire.

Sorry I'm jumping all over right now but it's early and my long asleep memories are coming alive on this one. I believe the manifolds have AIR injection systems with one way valves on them. Aspirator valves is what they're called. If they blow out you'll hear a fluttering noise and if they fail the tubing and plastic valves near them will melt. If the cats are plugged more than likely the valves are damaged and should be replaced. If you do need to replace them post it or call before you put wrenches on them and wait for a reply. One slip up and you may be removing manifolds.
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Re: 1981 Chev Caprice - 305V* Flat Cam

Post by Tim Martin »

Well, I dont know what the term "flat cam" means. I never heard it put that way but those 5.0L cams were known for a camshaft lobe wearing away to the point of a misfire. If the rocker covers are removed and you watch the rockers move as the engine is run the bad cam lobe can soon be spotted. However, in your case, I'd be checking for a plugged exhaust as well.

Could you describe the wierd noise? Is it a whistle/hissing type or a popping noise?
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brother bubba
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Re: 1981 Chev Caprice - 305V8 Flat Cam

Post by brother bubba »

....

I've seen a number of flat cams from that era. Stick a vacuum gage on the manifold. Telltale sign is fluctuating vacuum. From there pop the valve covers off and check the rockers for proper movement.

....
Measure it with a micrometer, mark it with a piece of chalk, cut it with an axe. Sand it to fit.
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Re: 1981 Chev Caprice - 305V8 Flat Cam

Post by jbadenoch »

I used to call them "butter cams" cause the lobes seemed to be made of it.
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Re: 1981 Chev Caprice - 305V8 Flat Cam

Post by sbebenelli »

Back in the day we replaced a lot of cams in 305 and 350's. The 305's were worst than the 350's.

If you rev it up and it cackles out the carburetor pull the valve covers off and see if some of the rockers aren't rocking like they should.
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