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Discussion Starter #1
I am sorting out a 68 Camaro with a 496 w/ aluminum heads and solid roller I just purchased. Previous owner had the valve lash set to around .023-.24 cold. I do not have the cam card at all just the specs of the cam itself. All my reading says that this is too loose and should probably set to .018 cold so that when hot it is .023-.024.

I went through and adjust all the valves based on the techniques from this site. E-O-I-C method. I also reset the Demon 850 carb to factory specs as well. I started the car up and had it idling at 500 rpms until I could get around to the front of the car and start adjusting the idle. I noticed a knock noise coming from engine maybe every 3-4 seconds. Seemed kinda random in intervals. I revved the motor and the knock went away, at idle you could hear it again. Instead of adjusting the idle, I turned the car off and went through the valves again and set them at .020 cold.

There are know issues with motor, I have driven it alot since I got it. Not sure if the low idle was accentuating the noise I was hearing or not. This is an awesome running motor but figured he has not set the valves in a while so thought I would go through them.

Previous owner states that he can hear an exhaust note at anything tighter then.023 lash setting. Not sure what that means or sounds like?

What should I do, start it again and adjust the idle up on both sides to get the idle to 900 rpms and see what it does?

I read on a Demon you have to set the prime and secondary idle the same amount of turns.

Thanks, Erik
 

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Modern cams have a tighter lash than the solid roller Crane cams sold by GM back in the seventies and late sixties. Crane made a lot of the OEM parts that Chevrolet sold; but it made them to GM specs, not something sold out of Harvey's cam catalog.

If you set a solid cam too tight it will beat the valve train to death and kill performance by keeping the valve from seating and sealing properly. A tighter lash will increase the effective lift of the cam to increase power but it is much safer and easier on the valve train to change the rocker arm ratio to do the same thing.

As mark has said, once you identify the cam grinder and the cam you have by way of identifying marks etched on the face (or unfortunately) on the heal of the cam (requiring you to either pull the cam out of the block or to drop the tranny and bell housing so you can pop out the rear Welch plug to look at the back side of the cam), you can check the numbers on line to find a cam card for that grind.

There should be at least 0.050 thousandths of an inch of piston to valve clearance but tightening the valves will reduce that clearance during valve over lap. I doubt if you will hit a piston with a valve IF THE MOTOR WAS ASSEMBLED PROPERLY. But then I didn't screw this one together so I would worry about any noise that is a function of how tight the valve lash is.

Larger Dave
 

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Discussion Starter #4
This is all the specs I have on the 496:

496 (454) iron block
Calles crank and rods
Bowtie aluminum cylinder heads ( I have part # at home)
Doug Herbert Billet solid roller cam (CB6K), int [email protected] - .668 lift, Exh. [email protected] - .680 lift (.018 Intake lash, .020 Exhaust lash)
Crower 1.7 stainless roller rockers
Titanium retainers
Super 7 keepers
Zero deck block 9:8.1 comp.

I was able to get ahold of Doug Herbert and he gave me the specs for cam.

I dont think this noise is valvetrain related, the valvetrain was moving rotating much faster then the noise coming from the motor. If was a the dreaded tick it would be consistent, this was kinda random.

Since it was idling so slow if that would have anything to do with. It was at 500 rpms when I heard the sound. When I revved it up just off idle it went away.
 

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A cam that big should never idle that low. With the overlap of that cam, the engine could do some strange things at that low of a speed. You could hear piston slap, or many things in the bottom end. I'm also assuming the oil pressure is rather low at 500 also.
 

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Discussion Starter #6
I was able to get running this evening and get the car idling at 900 rpms. Still had a noise coming from somewhere. The 500rpms was me starting the car, getting out of the car and getting to the carb to adjust the idle up to 900-1000 rpms and that is when i heard the noise.

I noticed tonight that some of the rockers were hitting the valve cover, I can see marks that I have not seen before. I am hoping this is as simple as a valve cover gasket change, the cork ones are pretty old and smashed.

Previous owner is adamant that this motor likes a .024 cold valve lash for some reason. Even though Doug Herbert says .018 to .020. Right now I have them set at .020. Not sure what to do there. Previous owner says anything less than .023 and you can here exhaust note, something about exhaust valve not closing all the way or something.

This is all new to me so I am learning as I go. This motor has some great parts in though. And it was fine until I touched it!!!
 

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Any update this morning Erik?
The new stuff coming from GM idles at 500rpm but your 496 probably won't be happy that low. The cam is not all that large for a 496. You should have a smooth idle when all is tuned. Your .018 to .020 setting (cold) will settle in, when hot, to be about .024 to .030. That's a bit sloppy for this day and age. With my L88 (now 468) and .725 lift, the cam card from Lunati gives a clearance spec of .025 OR LESS. And it doesn't say if that's hot or cold. So, some engines can use less clearance than others. (It's a 'combo' thing) When talking to the designer of my cam, HD Harold, he said I could set my valves, I&E, to about .009 cold. That should give about .015 hot. That's safe and will get the most out of the cam.
If your rockers are hitting the valve covers, that's a whole 'nuther problem. But easy to fix. Do that first and see what you've got.
That combo sounds great, by the way.
 

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Discussion Starter #8
I am out of town for the weekend so wont know until tomorrow evening. I definetaly think the noise was the rockers hitting the valve covers. I need new gaskets as the corks are smashed down. I reset everything to .020 and will start it up tomorrow night.

Thanks, Erik
 

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Every cam card I have ever seen is always a hot setting, never cold. My ls6 grew .006 from cold to hot. .012 cold grew to .018. Once I verified the growth rate, I always lashed the valves cold, very consistent method, as nothing changing during from the first to the last valve, plus who wants to deal with hot parts and oil.

950 rpm is your number for that cam at idle, any 4 corner carb should always be set equally on front and rear throttle plates.use manifold vacuum if you run a vacuum advance.
 
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