496 timing - Chevelle Tech
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post #1 of 12 (permalink) Old Oct 4th, 19, 10:39 AM Thread Starter
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Gary496
 
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496 timing

My 496 is timed at 18 degrees at 500 rpm with vacuum line plugged....engine at normal operating temp.
With the vacuum line hooked back up to full manifold vacuum port on the carb it idles at 900 rpm timing reads 30 degrees.

If I put the vacuum hose on the timed vacuum port on the carb...timing stays 18 degrees and idle stays at 500 rpm.
Makes no difference whether vacuum hose is hooked up or not hooked up. Shouldn't the idle increase with the vacuum hose hooked up to the timed port? Engine runs good on the full vacuum port on the carb but there is excessive unburned fuel smoke out the exhaust at idle speed. Carb is new Proform Race 850 (no choke assembly) double pump.
Running too rich at idle. Carb too big? Adjusted the carb still have too much unburned fuel. Does it need hotter plugs? Would more timing help burn the fuel? Run full vacuum or ported vacuum?
Engine starts fine at full vacuum no heat soak sound like you you hear if it has too much initial timing or weak battery and no dieseling when you shut off the engine. Running MSD ignition.... dist, 6A box, new plug wires and plugs.
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post #2 of 12 (permalink) Old Oct 4th, 19, 11:13 AM
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Re: 496 timing

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Originally Posted by 70BBC496 View Post
Shouldn't the idle increase with the vacuum hose hooked up to the timed port?
No. Its above the throttle plates. Once the throttle is open,that is when the port will pull vacuum.


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Originally Posted by 70BBC496 View Post
Running too rich at idle
These cars will never idle truly "clean", nature of the beast.especially with a camshaft that has a lot of overlap.

Sounds like your timing curve is pretty decent,I bet your car runs well and turns on nice when you drop the hammer


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post #3 of 12 (permalink) Old Oct 4th, 19, 11:41 AM
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Re: 496 timing

Carb?
What plugs(heat range)?
What cam hyd solid flat tappet or roller?
Spec?
Try manifold vacuum on the vacuum advance and limit the vacuum advance to 10-12degree.
Is it a msd 8361 distributor? You can get bushings with 10 or 14 degree the mech advance if you want.

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post #4 of 12 (permalink) Old Oct 4th, 19, 1:38 PM Thread Starter
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Re: 496 timing

NGK BKR5E plugs. Cam hyd. roller lift 618/[email protected] 234/240.

MSD 8361 dist.
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post #5 of 12 (permalink) Old Oct 4th, 19, 1:45 PM
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Re: 496 timing

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Originally Posted by 70BBC496 View Post
NGK BKR5E plugs. Cam hyd. roller lift 618/[email protected] 234/240.

MSD 8361 dist.
Ok those plugs should work and be correct in heat range imho.
With that cam it will like more timing at idle try to hook up the vacuum advance to manifold vacuum and you probably need to limit its travel so it dont give you more than 10-12 deg.

If needed you can get 10 and 14 deg bushings for that distributor.

Chevelle Malibu ss 64
489 BB with parts from Lewis racing
Alumitech radiator with dual spals
Th 400 Edge 9.5" converter
57 3/4 wide 12 bolt: Eaton posi, Us gear 3.31, Toms KA 33 splines axles
Malibu ss 64 is online now  
post #6 of 12 (permalink) Old Oct 4th, 19, 1:48 PM Thread Starter
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Re: 496 timing

Proform has both of the vacuum ports side by side in the front base of the carb. The one closest to the the outside is full and inside one is timed.
That is what they told me when I called Proform. They don't include any instructions in the box. NOTHING.
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post #7 of 12 (permalink) Old Oct 4th, 19, 2:37 PM Thread Starter
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Re: 496 timing

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post #8 of 12 (permalink) Old Oct 4th, 19, 4:11 PM
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Re: 496 timing

EASIEST way to tell if the port is ported or full manifold vacuum is to stock a finger over the port, at IDLE< and see if it has vacuum. NO vacuum, ported port. Strong idle vacuum, full manifold vacuum port. Simple as that.

Now, we see yet anther engine over initial timed, another Top Fuel dragster engine on the street. Instead of 18 degrees, run the initial down to 14, and add the vacuum advance with its 12 degrees to full manifold vacuum, for a total IDLE timing of 26. That cam has a very late intake valve closing, and a DCR that will tolerate the 26 degrees, one heck of a lot better than the full race 18 and no vacuum advance 30 idle.

ONLY thing you might have to do if you don't have an adjustable vacuum advance, is to get one, use the Crane/FAST, and install the degrees stop plate on the OTHER side of the pull pin, NOT where Crane says to. If you need a FREE complete set of instructions on how to do the stop plate, ask for them,

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post #9 of 12 (permalink) Old Oct 5th, 19, 12:16 PM Thread Starter
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Re: 496 timing

Set the timing to 14 degrees with vacuum advance unplugged and the hose to full manifold plugged off?
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post #10 of 12 (permalink) Old Oct 5th, 19, 12:57 PM
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Re: 496 timing

WHAT? 14 and leave the vacuum advance plugged off????? WRONG, WRONG, WRONG, AND...WRONG AGAIN. 14 INITIAL, add the 10/12 vacuum advance degrees from the full manifold vacuum source port, and that will give you 24 to 26 degrees IDLE timing.

Get the instructions, and add the Crane adjustable degrees stop, which makes vacuum advance degrees setting so very easy.
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post #11 of 12 (permalink) Old Oct 5th, 19, 1:01 PM
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Re: 496 timing

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Originally Posted by 70BBC496 View Post
Set the timing to 14 degrees with vacuum advance unplugged and the hose to full manifold plugged off?
Yes....then plug the vacuum advance in to the manifold vacuum. Check the timing again and you should have 10-12 in addition to the 14 you started out with...

72 SS 402/4 speed/Straub roller and a bunch of Duramaxes.....
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post #12 of 12 (permalink) Old Oct 5th, 19, 3:52 PM Thread Starter
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Re: 496 timing

OK.....got it.....thanks guys!
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