383 Vortec Holley carb rough idle question - Hot Rod Forum : Hotrodders Bulletin Board
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Old 06-02-2008, 08:50 AM
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383 Vortec Holley carb rough idle question

Here is the setup: 70 Impala 383 with Vortec heads, comp 268xe cam, mallory hei @ 10 initial, holley 600 w/ vac secs, 200-4R w/ 1800-2K stall, 3.55 gears.

I'm having trouble setting the idle. I borrowed a vac gauge to set the mixture screws, pulled the cap off the bottom manifold vac port, and noticed that it dripped fuel. Don't know what this means, but I've never had this happen on a carb before (small red flag?). Started the motor, and I got around 15" @ 2.5 turns out. Set everything for 850 rpm, and things seem fine, put it in gear and it wants to die. I checked the float level before I started, and I thought things looked good (small drip, not pouring out).

Took the idle up high enough where it wouldn't die in gear (~1200 in neutral). Taking off it wanted to dog until I was past 1500. Too much fuel maybe?

Any help is much appreciated!

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Old 06-02-2008, 05:33 PM
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you need more timing. try 15 or 16 degrees initial.

Then connect the vacuum advance to the manifold vacuum. you will run around 30 degrees at idle.

lower the fuel bowl. dripping fuel is no good.

are you sure the timing marks are right? there are two different timing mark styles there are not interchangeable.
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Old 06-02-2008, 09:51 PM
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The fuel level in the bowls appears to be correct. It appears to be getting fuel into the manifold-vacuum port on the front corner of the carb. I also think the vacuum advance on the HEI may be coming in at idle rpm, what could cause this? It is all stock. There is definitely vac on the distributor port at idle.
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Old 06-03-2008, 08:13 AM
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If there is vacuum on the ported vacuum source at idle then the throttle blades are open too far at idle. This means you are into the main metering circuit at idle which is like trying to fill a shot glass with a fire hose.

Blades opened too much at idle typically indicates the carb is too small for the engine.

There are a few possible cures. One is to add more timing which will increase the idle which then will allow you to close the throttle blades more at idle. The other thing you can do is to let more air in by cracking open the rear secondary blades with the set screw on the bottom of the carb or you could drill small holes the front throttle blades. I would try adding more timing first.

Unplug the vacuum advance and set the timing at 16 degrees and then plug the distributor's vacuum advance to a manifold vacuum source. Total timing at idle should be in the high 20's or low 30's with both the mechanical and vacuum added together. Then turn the idle down to 850 rpms and check if there is vacuum on the ported source. If there isn't vacuum on the ported source then the throttle blades are closed enough. then set idle screws for best idle.

Vacuum advance on the ported sided is a emissions setup. lower timing at idle makes the exhaust hotter which burns up the bad stuff. You are are way past the emission setup so use the manifold vacuum source for your distributor
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Old 06-03-2008, 08:26 AM
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Thanks for the info. Can you elaborate on why the throttle blades open means the carb is too small?

16 initial is way more than I have seen for timing with vortec heads, not that I wouldn't try it, but I was looking at what GM recommended for it's crate engines that are similar and thats were the 10 came from.
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Old 06-03-2008, 08:46 AM
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The engine needs a set amount of air to idle with. Your 600 cfm carb is having a hard time delivering enough air with the blades in the idle circuit. If the blades open past a point, the main circuit takes over and pours in fuel as if your were driving.

Adding more timing makes the idle speed increase. More idle from timing allows you to shut down the blades.

The cam you are using dictates the idle ignition timing. Sure, vortec heads require less total timing but if you are running a lopey cam, then you need more timing. And a 268xe cam needs more timing.

Your mallory hei should have adjustment in the curve. set total advance to 36 and initial to 16. use the vacuum advance on the manifold vacuum but set the timing with it unplugged.

Again, are you sure the timing marks are right??
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Old 06-03-2008, 09:01 AM
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Good Carb tuning links

http://holley.com/TechService/Instructions.asp

http://holley.com/data/Products/Tech...199R7948-5.pdf

http://www.bob2000.com/carb.htm
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Old 06-03-2008, 10:45 AM
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Thanks for the links sedan64, and thanks for clearing up the timing/cam relationship 454c10.

I am pretty confident that the timing mark is correct. I am using a dialed timing light, just set to X and dial in the center mark on the 0 mark.
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Old 06-03-2008, 11:01 AM
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I'm talking about making sure the timing mark on the timing chain cover is correct for the mark placement on the balancer. There are two versions out there and they are not compatible. So, it maybe showing 10 degrees with the timing light but it is actually something else due to mark placement error.

You can check to true TDC with a piston stop.
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Old 06-03-2008, 02:26 PM
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I understand there are two styles. I'm pretty sure I have the correct one, or at least I'll take the chance for now
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Old 06-03-2008, 04:44 PM
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more links

http://www.mortec.com/carbtip1.htm

http://www.mortec.com/carbs.htm
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Old 06-04-2008, 08:01 PM
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I have figured out the idle to the best of my abilities, thanks to the help of everyone here. I moved to 16* initial, and I ended up taking a turn on the secondary throttle screw so I could get a little more air. Now it is holding pretty evenly and not dying when I put it in gear.
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